Semiconductor device and display device

ABSTRACT

In a transistor including an oxide semiconductor film, field-effect mobility and reliability are improved. A semiconductor device includes a gate electrode, an insulating film over the gate electrode, an oxide semiconductor film over the insulating film, and a pair of electrodes over the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film include the same elements. The second oxide semiconductor film includes a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.

2. Description of the Related Art

As a semiconductor material that can be used in a transistor, an oxide semiconductor has been attracting attention. For example, Patent Document 1 discloses a semiconductor device whose field-effect mobility (in some cases, simply referred to as mobility or μFE) is improved by stacking a plurality of oxide semiconductor layers, among which the oxide semiconductor layer serving as a channel contains indium and gallium where the proportion of indium is higher than the proportion of gallium.

Non-Patent Document 1 discloses that an oxide semiconductor containing indium, gallium, and zinc has a homologous series represented by In_(1-x)Ga_(1+x)O₃(ZnO)_(m) (x is a number which satisfies −1≦x≦1, and m is a natural number). Furthermore, Non-Patent Document 1 discloses a solid solution range of a homologous series. For example, in the solid solution range of the homologous series in the case where m=1, x ranges from −0.33 to 0.08, and in the solid solution range of the homologous series in the case where m=2, x ranges from −0.68 to 0.32.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2014-007399

Non-Patent Document

-   [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The     Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.     Solid State Chem., Vol. 93, 1991, pp. 298-315.

SUMMARY OF THE INVENTION

The field-effect mobility of a transistor that uses an oxide semiconductor film as a channel region is preferably as high as possible. However, when the field-effect mobility is increased, the transistor has a problem with its characteristics, that is, the transistor tends to be normally on. Note that “normally on” means a state where a channel exists without application of a voltage to a gate electrode and a current flows through the transistor.

Furthermore, in a transistor that uses an oxide semiconductor film in a channel region, oxygen vacancies which are formed in the oxide semiconductor film adversely affect the transistor characteristics. For example, oxygen vacancies formed in the oxide semiconductor film are bonded with hydrogen to serve as a carrier supply source. The carrier supply source generated in the oxide semiconductor film causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor including the oxide semiconductor film.

Too many oxygen vacancies in an oxide semiconductor film, for example, shift the threshold voltage of the transistor in the negative direction, causing normally-on characteristics. Thus, it is preferable that a channel region in an oxide semiconductor film especially include few oxygen vacancies or include oxygen vacancies such that normally-on characteristics are not caused.

In view of the foregoing problems, an object of one embodiment of the present invention is to improve field-effect mobility and reliability in a transistor including an oxide semiconductor film. Another object of one embodiment of the present invention is to prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and to improve reliability of the transistor. Another object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all of these objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.

An embodiment of the present invention is a semiconductor device which includes a gate electrode, an insulating film over the gate electrode, an oxide semiconductor film over the insulating film, and a pair of electrodes over the oxide semiconductor film. In the semiconductor device, the oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film include the same elements, and the second oxide semiconductor film includes a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.

An embodiment of the present invention is a semiconductor device which includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a pair of electrodes over the oxide semiconductor film, a second insulating film over the oxide semiconductor film and the pair of electrodes, and a second gate electrode over the second insulating film.

In the semiconductor device, the oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film include the same elements. The second oxide semiconductor film includes a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.

An embodiment of the present invention is a semiconductor device which includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a pair of electrodes over the oxide semiconductor film, a second insulating film over the oxide semiconductor film and the pair of electrodes, and a second gate electrode over the second insulating film. In the semiconductor device, the first gate electrode and the second gate electrode are connected through an opening provided in the first insulating film and the second insulating film. The first gate electrode and the second gate electrode each include a region positioned outside an edge portion of the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film include the same elements. The second oxide semiconductor film includes a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.

In the above embodiments, the second oxide semiconductor film preferably includes nitrogen. Furthermore, in the above embodiments, the second oxide semiconductor film preferably includes a region having a higher nitrogen concentration than the first oxide semiconductor film and the third oxide semiconductor film.

In the above embodiments, it is preferable that the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately include indium, M (M is aluminum, gallium, yttrium, or tin), and zinc. Furthermore, in the above embodiments, it is preferable that the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately include a crystal part which has a c-axis alignment.

An embodiment of the present invention is a semiconductor device which includes a first insulating film, an oxide semiconductor film over the first insulating film, a second insulating film and a third insulating film over the oxide semiconductor film, and a gate electrode over the second insulating film. In the semiconductor device, the oxide semiconductor film includes a first region in contact with the first insulating film, a second region in contact with the second insulating film, and a third region in contact with the third insulating film. The first region includes a region having a higher carrier density than the second region. The third region includes a region having a higher carrier density than the second region.

An embodiment of the present invention is a semiconductor device which includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a second insulating film and a third insulating film over the oxide semiconductor film, and a second gate electrode over the second insulating film. In the semiconductor device, the oxide semiconductor film includes a first region in contact with the first insulating film, a second region in contact with the second insulating film, and a third region in contact with the third insulating film. The first region includes a region having a higher carrier density than the second region. The third region includes a region having a higher carrier density than the second region.

An embodiment of the present invention is a semiconductor device which includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a second insulating film and a third insulating film over the oxide semiconductor film, and a second gate electrode over the second insulating film. In the semiconductor device, the first gate electrode and the second gate electrode are connected through an opening provided in the first insulating film and the second insulating film. The first gate electrode and the second gate electrode each include a region positioned outside an edge portion of the oxide semiconductor film. The oxide semiconductor film includes a first region in contact with the first insulating film, a second region in contact with the second insulating film, and a third region in contact with the third insulating film. The first region includes a region having a higher carrier density than the second region, and the third region includes a region having a higher carrier density than the second region.

In the above embodiments, the first region preferably includes nitrogen. In the above embodiments, the first region preferably includes a region having a higher nitrogen concentration than the second region.

In the above embodiments, the oxide semiconductor film preferably includes indium, M (M is aluminum, gallium, yttrium, or tin), and zinc. In the above embodiments, the oxide semiconductor film preferably includes a crystal part which has a c-axis alignment.

Another embodiment of the present invention is a display device which includes the semiconductor device according to any one of the above-described embodiments, and a display element. Another embodiment of the present invention is a display module which includes the display device and a touch sensor. Another embodiment of the present invention is an electronic device which includes the semiconductor device according to any one of the above-described embodiments, the above-described display device, or the above-described display module; and an operation key or a battery.

One embodiment of the present invention can improve field-effect mobility and reliability in a transistor including an oxide semiconductor film. One embodiment of the present invention can prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and improve the reliability of the transistor. One embodiment of the present invention can provide a semiconductor device with low power consumption. One embodiment of the present invention can provide a novel semiconductor device. One embodiment of the present invention can provide a novel display device.

Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily have all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views illustrating a semiconductor device;

FIGS. 2A to 2C are a top view and cross-sectional views illustrating a semiconductor device;

FIGS. 3A to 3C are a top view and cross-sectional views illustrating a semiconductor device;

FIGS. 4A to 4C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 5A to 5C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 6A to 6C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 7A to 7C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 8A to 8C are a top view and cross-sectional views illustrating a semiconductor device;

FIGS. 9A to 9C are a top view and cross-sectional views illustrating a semiconductor device;

FIGS. 10A and 10B are cross-sectional views illustrating a semiconductor device;

FIGS. 11A and 11B are cross-sectional views illustrating a semiconductor device;

FIGS. 12A to 12D are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 13A to 13C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 14A to 14C are cross-sectional views illustrating a method for manufacturing a semiconductor device;

FIGS. 15A to 15C each illustrate the range of an atomic ratio of an oxide semiconductor;

FIG. 16 illustrates an InMZnO₄ crystal;

FIGS. 17A to 17E show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD and selected-area electron diffraction patterns of a CAAC-OS;

FIGS. 18A to 18E show a cross-sectional TEM image and plan-view TEM images of a CAAC-OS and images obtained through analysis thereof;

FIGS. 19A to 19D show electron diffraction patterns and a cross-sectional TEM image of an nc-OS;

FIGS. 20A and 20B show cross-sectional TEM images of an a-like OS;

FIG. 21 shows a change in crystal part of an In—Ga—Zn oxide induced by electron irradiation;

FIG. 22 is a top view illustrating one embodiment of a display device;

FIG. 23 is a cross-sectional view illustrating one embodiment of a display device;

FIG. 24 is a cross-sectional view illustrating one embodiment of a display device;

FIG. 25 is a cross-sectional view illustrating one embodiment of a display device;

FIG. 26 is a cross-sectional view illustrating one embodiment of a display device;

FIG. 27 is a cross-sectional view illustrating one embodiment of a display device;

FIGS. 28A to 28D are cross-sectional views illustrating a method for forming an EL layer;

FIG. 29 is a conceptual diagram illustrating a droplet discharge apparatus;

FIGS. 30A to 30C are a block diagram and circuit diagrams illustrating a display device;

FIGS. 31A to 31C are circuit diagrams and a timing chart illustrating one embodiment of the present invention;

FIGS. 32A to 32C are a graph and circuit diagrams illustrating one embodiment of the present invention;

FIGS. 33A and 33B are a circuit diagram and a timing chart illustrating one embodiment of the present invention;

FIGS. 34A and 34B are a circuit diagram and a timing chart illustrating one embodiment of the present invention;

FIGS. 35A to 35E are a block diagram, circuit diagrams, and waveform diagrams illustrating one embodiment of the present invention;

FIGS. 36A and 36B are a circuit diagram and a timing chart illustrating one embodiment of the present invention;

FIGS. 37A and 37B are circuit diagrams illustrating one embodiment of the present invention;

FIGS. 38A to 38C are circuit diagrams illustrating one embodiment of the present invention;

FIG. 39 illustrates a display module;

FIGS. 40A to 40E illustrate electronic devices;

FIGS. 41A to 41G illustrate electronic devices; and

FIGS. 42A and 42B are perspective views illustrating a display device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings. However, the embodiments can be implemented in many different modes, and it will be readily appreciated by those skilled in the art that modes and details thereof can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments.

In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings.

Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, such as “over” “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. Further, the positional relation between components is changed as appropriate in accordance with a direction in which the components are described. Thus, the positional relation is not limited to that described with a term used in this specification and can be explained with another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow through the drain region, the channel region, and the source region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Further, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.

Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

In this specification and the like, the term “parallel” means that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 100, and accordingly also covers the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” means that the angle formed between two straight lines is greater than or equal to 800 and less than or equal to 1000, and accordingly also covers the case where the angle is greater than or equal to 85° and less than or equal to 950°.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

Unless otherwise specified, the off-state current in this specification and the like refers to a drain current of a transistor in the off state (also referred to as non-conduction state and cutoff state). Unless otherwise specified, the off state of an n-channel transistor means that a voltage (V_(gs)) between its gate and source is lower than the threshold voltage (V_(th)), and the off state of a p-channel transistor means that the gate-source voltage V_(gs) is higher than the threshold voltage V_(th). For example, the off-state current of an n-channel transistor sometimes refers to a drain current that flows when the gate-source voltage V_(gs) is lower than the threshold voltage V_(th).

The off-state current of a transistor depends on V_(gs) in some cases. Thus, “the off-state current of a transistor is lower than or equal to I” may mean “there is V_(gs) with which the off-state current of the transistor becomes lower than or equal to 1”. Furthermore, “the off-state current of a transistor” means “the off-state current in an off state at predetermined V_(gs)”, “the off-state current in an off state at V_(gs) in a predetermined range”, “the off-state current in an off state at V_(gs) with which sufficiently reduced off-state current is obtained”, or the like.

As an example, the assumption is made of an n-channel transistor where the threshold voltage V_(th) is 0.5 V and the drain current is 1×10⁻⁹ A at V_(gs) of 0.5 V, 1×10⁻¹³ A at V of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of −0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of the transistor is 1×10⁻¹⁹ A or lower at V, of −0.5 V or at V_(gs), in the range of −0.8 V to −0.5 V; therefore, it can be said that the off-state current of the transistor is 1×10⁻¹⁹ A or lower. Since there is V_(gs) at which the drain current of the transistor is 1×10⁻²² A or lower, it may be said that the off-state current of the transistor is 1×10⁻²² A or lower.

In this specification and the like, the off-state current of a transistor with a channel width W is sometimes represented by a current value in relation to the channel width W or by a current value per given channel width (e.g., 1 μm). In the latter case, the off-state current may be expressed in the unit with the dimension of current per length (e.g., A/μm).

The off-state current of a transistor depends on temperature in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at room temperature, 60° C., 85° C., 95° C., or 125° C. Alternatively, the off-state current may be an off-state current at a temperature at which the reliability required in a semiconductor device or the like including the transistor is ensured or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.). The description “an off-state current of a transistor is lower than or equal to I” may refer to a situation where there is V_(gs) at which the off-state current of a transistor is lower than or equal to I at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature at which the reliability required in a semiconductor device or the like including the transistor is ensured, or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.).

The off-state current of a transistor depends on voltage V_(ds) between its drain and source in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at V_(ds) of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V. Alternatively, the off-state current might be an off-state current at V_(ds) at which the required reliability of a semiconductor device or the like including the transistor is ensured or V_(ds) at which the semiconductor device or the like including the transistor is used. The description “an off-state current of a transistor is lower than or equal to I” may refer to a situation where there is V_(gs) at which the off-state current of a transistor is lower than or equal to I at V_(ds) of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, V_(ds) at which the required reliability of a semiconductor device or the like including the transistor is ensured, or V_(gs) at which in the semiconductor device or the like including the transistor is used.

In the above description of off-state current, a drain may be replaced with a source. That is, the off-state current sometimes refers to a current that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimes expresses the same meaning as off-state current. In this specification and the like, the off-state current sometimes refers to a current that flows between a source and a drain when a transistor is off, for example.

In this specification and the like, the threshold voltage of a transistor refers to a gate voltage (V_(g)) at which a channel is formed in the transistor. Specifically, in a graph where the horizontal axis represents the gate voltage (V_(g)) and the vertical axis represents the square root of drain current (I_(d)), the threshold voltage of a transistor may refer to a gate voltage (V_(g)) at the intersection of the square root of drain current (I_(d)) of 0 (I_(d)=0 A) and an extrapolated straight line that is tangent with the highest inclination to a plotted curve (V_(g)−√I_(d) characteristics). Alternatively, the threshold voltage of a transistor may refer to a gate voltage (V_(g)) at which the value of I_(d) [A]×L [μm]/W [μm] is 1×10⁻⁹ [A] where L is channel length and W is channel width.

In this specification and the like, a “semiconductor” can have characteristics of an “insulator” when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called an “insulator” in some cases. Similarly, an “insulator” in this specification and the like can be called a “semiconductor” in some cases. An “insulator” in this specification and the like can be called a “semi-insulator” in some cases.

In this specification and the like, a “semiconductor” can have characteristics of a “conductor” when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called a “conductor” in some cases. Similarly, a “conductor” in this specification and the like can be called a “semiconductor” in some cases.

In this specification and the like, an impurity in a semiconductor refers to an element that is not a main component of the semiconductor film. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. If a semiconductor contains an impurity, the density of states (DOS) may be formed therein, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. In the case where the semiconductor includes an oxide semiconductor, examples of the impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specific examples are hydrogen (also included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. When the semiconductor is an oxide semiconductor, oxygen vacancies may be formed by entry of impurities such as hydrogen, for example. Furthermore, in the case where the semiconductor includes silicon, examples of the impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.

In this specification and the like, a metal oxide means an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor (also simply referred to as an OS), and the like. For example, a metal oxide used in an active layer of a transistor is called an oxide semiconductor in some cases. In other words, an OS FET is a transistor including a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide including nitrogen is also called a metal oxide in some cases. Moreover, a metal oxide including nitrogen may be called a metal oxynitride.

In this specification and the like, “c-axis aligned crystal (CAAC)” or “cloud-aligned composite (CAC)” might be stated. CAAC refers to an example of a crystal structure, and CAC refers to an example of a function or a material composition.

An example of a crystal structure of an oxide semiconductor or a metal oxide is described. Note that an oxide semiconductor deposited by a sputtering method using an In—Ga—Zn oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio) is described below as an example. An oxide semiconductor formed by a sputtering method using the above-mentioned target at a substrate temperature of higher than or equal to 100° C. and lower than or equal to 130° C. is referred to as sIGZO, and an oxide semiconductor formed by a sputtering method using the above-mentioned target with the substrate temperature set at room temperature (R.T.) is referred to as tIGZO. For example, sIGZO has one or both crystal structures of nano crystal (nc) and CAAC. Furthermore, tIGZO has a crystal structure of nc. Note that room temperature (R.T.) herein also refers to a temperature of the time when a substrate is not heated intentionally.

In this specification and the like, CAC-OS or CAC-metal oxide has a function of a conductor in a part of the material and has a function of a dielectric (or insulator) in another part of the material; as a whole, CAC-OS or CAC-metal oxide has a function of a semiconductor. In the case where CAC-OS or CAC-metal oxide is used in an active layer of a transistor, the conductor has a function of letting electrons (or holes) serving as carriers flow, and the dielectric has a function of not letting electrons serving as carriers flow. By the complementary action of the function as a conductor and the function as a dielectric, CAC-OS or CAC-metal oxide can have a switching function (on/off function). In the CAC-OS or CAC-metal oxide, separation of the functions can maximize each function.

In this specification and the like, CAC-OS or CAC-metal oxide includes conductor regions and dielectric regions. The conductor regions have the above-described function of the conductor, and the dielectric regions have the above-described function of the dielectric. In some cases, the conductor regions and the dielectric regions in the material are separated at the nanoparticle level. In some cases, the conductor regions and the dielectric regions are unevenly distributed in the material. When observed, the conductor regions are coupled in a cloud-like manner with their boundaries blurred, in some cases.

In other words, CAC-OS or CAC-metal oxide can be called a matrix composite or a metal matrix composite.

Furthermore, in the CAC-OS or CAC-metal oxide, the conductor regions and the dielectric regions each have a size of more than or equal to 0.5 nm and less than or equal to 10 nm, preferably more than or equal to 0.5 nm and less than or equal to 3 nm and are dispersed in the material, in some cases.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention and a manufacturing method of the semiconductor device are described with reference to FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6C, and FIGS. 7A to 7C.

<1-1. Structural Example 1 of Semiconductor Device>

FIG. 1A is a plan view of a transistor 100 that is a semiconductor device of one embodiment of the present invention. FIG. 1B is a cross-sectional view taken along a dashed dotted line X1-X2 in FIG. 1A, and FIG. 1C is a cross-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 1A. Note that in FIG. 1A, some components of the transistor 100 (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. Furthermore, the direction of the dashed dotted line X1-X2 may be referred to as a channel length direction, and the direction of the dashed dotted line Y1-Y2 may be referred to as a channel width direction. As in FIG. 1A, some components are not illustrated in some cases in plan views of transistors described below.

A transistor 100 includes a conductive film 104 over a substrate 102, an insulating film 106 over the substrate 102 and the conductive film 104, an oxide semiconductor film 108 over the insulating film 106, a conductive film 112 a over the oxide semiconductor film 108, and a conductive film 112 b over the oxide semiconductor film 108. Furthermore, an insulating film 114, an insulating film 116 over the insulating film 114, and an insulating film 118 over the insulating film 116 are formed over the transistor 100, specifically over the oxide semiconductor film 108, the conductive film 112 a, and the conductive film 112 b.

Note that the transistor 100 is what is called a channel-etched transistor.

Furthermore, the oxide semiconductor film 108 includes an oxide semiconductor film 108 i_1 over the insulating film 106, an oxide semiconductor film 108 n over the oxide semiconductor film 108 i_1, and an oxide semiconductor film 108 i_2 over the oxide semiconductor film 108 n. Note that the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 include the same elements. For example, it is preferable that the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 each independently include In, M (M is Al, Ga, Y, or Sn), and Zn.

The oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 preferably each independently include a region in which the atomic proportion of In is larger than the atomic proportion of M.

For example, the atomic ratio of In, M, and Zn in the oxide semiconductor film 108 i_1 is preferably In:M:Zn=4:2:3 or in the vicinity thereof. Further, the atomic ratio of In, M. and Z in the oxide semiconductor film 108 n is preferably In:M:Zn=4:2:3 or in the vicinity thereof. In addition, the atomic ratio of In, M, and Z in the oxide semiconductor film 108 i_2 is preferably In:M:Zn=4:2:3 or in the vicinity thereof. As for the range expressed by the term “vicinity” here, when In is 4, M ranges from 1.5 to 2.5 and Zn ranges from 2 to 4.

When the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 each independently include a region in which the atomic proportion of In is larger than the atomic proportion of M, the transistor 100 can have high field-effect mobility. Specifically, the field-effect mobility of the transistor 100 can exceed 10 cm²/Vs, preferably exceed 30 cm²/Vs.

For example, the use of the transistor with high field-effect mobility in a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame.

When the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 each independently include a region in which the atomic proportion of In is larger than the atomic proportion of M, on the other hand, electric characteristics, e.g., the threshold, of the transistor 100 may vary in the gate bias temperature stress (GBT) test.

However, in the semiconductor device of one embodiment of the present invention, the oxide semiconductor film 108 n includes a region having a higher carrier density than the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2.

The increased carrier density in the oxide semiconductor film 108 n can increase the Fermi level relative to the conduction band of the oxide semiconductor film 108 n. This lowers the conduction band minimum of the oxide semiconductor film 108 n, so that the energy difference between the conduction band minimum of the oxide semiconductor film 108 n and the trap level, which might be formed in a gate insulating film (here, the insulating film 106), can be increased. The increase of the energy difference can reduce trap of charges in the gate insulating film and reduce variation in the threshold voltage of the transistor in the GBT test, in some cases.

Impurities such as hydrogen or moisture entering the oxide semiconductor film 108 adversely affect the transistor characteristics and therefore cause a problem. Therefore, it is preferable that the amount of impurities such as hydrogen or moisture in the oxide semiconductor film 108, particularly in the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2, be as small as possible. Furthermore, oxygen vacancies formed in the oxide semiconductor film 108 adversely affect the transistor characteristics and therefore cause a problem. For example, oxygen vacancies formed in the oxide semiconductor film 108 are bonded to hydrogen to serve as a carrier supply source. The carrier supply source generated in the oxide semiconductor film 108 causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor 100 including the oxide semiconductor film 108. Therefore, it is preferable that the amount of oxygen vacancies in the oxide semiconductor film 108, particularly in the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2, be as small as possible.

In view of this, one embodiment of the present invention is a structure in which insulating films in the vicinity of the oxide semiconductor film 108, specifically the insulating films 114 and 116 formed over the oxide semiconductor film 108, include excess oxygen. Oxygen or excess oxygen is transferred from the insulating films 114 and 116 to the oxide semiconductor film 108, whereby the oxygen vacancies in the oxide semiconductor film can be reduced.

Note that it is preferable to use, as the oxide semiconductor film 108, an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

For example, the carrier density of the oxide semiconductor film 108 is preferably higher than or equal to 1×10⁵ cm⁻³ and lower than 1×10¹⁸ cm⁻³, further preferably higher than or equal to 1×10⁷ cm⁻³ and lower than or equal to 1×10¹⁷ cm⁻³, still further preferably higher than or equal to 1×10⁹ cm⁻³ and lower than or equal to 5×10¹⁶ cm⁻³, yet further preferably higher than or equal to 1×10¹⁰ cm⁻³ and lower than or equal to 1×10¹⁶ cm⁻³, and yet still preferably higher than or equal to 1×10¹³ cm⁻³ and lower than or equal to 1×10¹⁵ cm⁻³.

One embodiment of the present invention employs a three-layer structure of the oxide semiconductor film 108 including the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 in order to increase the carrier density of the oxide semiconductor film 108. The oxide semiconductor film 108 n includes a region having a higher carrier density than the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2.

The carrier density of the oxide semiconductor film 108 n can be, for example, increased by making the amount of oxygen vacancies or the impurity concentration in the oxide semiconductor film 108 n slightly higher than those of the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2.

To increase the carrier density of the oxide semiconductor film 108 n, an element that forms oxygen vacancies may be added into the oxide semiconductor film 108 n so that hydrogen or the like can be bonded to the oxygen vacancies. Typical examples of the element that forms oxygen vacancies include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, and a rare gas element. Typical examples of the rare gas element are helium, neon, argon, krypton, and xenon. Note that among the above-mentioned elements, nitrogen is particularly preferable as the element that forms oxygen vacancies in the oxide semiconductor film.

For example, by using argon gas and dinitrogen monoxide gas as a deposition gas in forming the oxide semiconductor film 108 n, nitrogen can be contained in the oxide semiconductor film 108 n. In this case, the oxide semiconductor film 108 n includes a region having a higher nitrogen concentration than the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2.

Accordingly, the oxide semiconductor film 108 n has a higher carrier density and is of slightly n-type. An oxide semiconductor film having a higher carrier density is described as “slightly-n”, in some cases.

For example, in the case where the voltage applied to the gate of the transistor (V_(g)) is higher than 0 V and lower than or equal to 30 V, the carrier density of the oxide semiconductor film 108 n is preferably higher than 1×10¹⁶ cm⁻³ and lower than 1×10¹⁸ cm⁻³, further preferably higher than 1×10¹⁶ cm⁻³ and lower than or equal to 1×10¹⁷ cm⁻³.

In the case where the carrier density of the oxide semiconductor film 108 n is increased, the crystallinity of the oxide semiconductor film 108 n is lower than those of the oxide semiconductor film 108 i_1 and the oxide semiconductor film 108 i_2, in some cases. In this case, the oxide semiconductor film 108 has a layered structure including a high-crystallinity oxide semiconductor film, a low-crystallinity oxide semiconductor film, and a high-crystallinity oxide semiconductor film. Since the crystallinity of an oxide semiconductor film has a correlation to the film density of the oxide semiconductor film, the oxide semiconductor film having higher crystallinity has a higher film density. Thus, the oxide semiconductor film 108 has a layered structure including an oxide semiconductor film having a high film density, an oxide semiconductor film having a low film density, and an oxide semiconductor film having a high film density.

Note that the crystallinity of the oxide semiconductor film 108 can be determined by analysis by X-ray diffraction (XRD) or with a transmission electron microscope (TEM). Furthermore, the film density of the oxide semiconductor film 108 can be measured with an X-ray reflectometer (XRR), for example.

As described above, the semiconductor device of one embodiment of the present invention employs a three-layer structure of an oxide semiconductor film where the middle layer in which carriers flow has a higher carrier density than the upper and lower layers because of having more oxygen vacancies or a higher impurity concentration. Moreover, in the three-layer structure of the oxide semiconductor film 108, the upper and lower layers include less oxygen vacancies and have lower impurity concentrations than the middle layer. In this way, the improvement in both the field-effect mobility and reliability of the transistor can be achieved in the semiconductor device of one embodiment of the present invention.

In the transistor 100 illustrated in FIGS. 1A to 1C, the insulating film 106 functions as a gate insulating film of the transistor 100, and the insulating films 114, 116, and 118 function as protective insulating films of the transistor 100. Furthermore, in the transistor 100, the conductive film 104 functions as a gate electrode, the conductive film 112 a functions as a source electrode, and the conductive film 112 b functions as a drain electrode. Note that in this specification and the like, in some cases, the insulating film 106 is referred to as a first insulating film, the insulating films 114 and 116 are collectively referred to as a second insulating film, and the insulating film 118 is referred to as a third insulating film.

<1-2. Components of Semiconductor Device>

Next, components of the semiconductor device of this embodiment are described in detail.

[Substrate]

There is no particular limitation on the property of a material and the like of the substrate 102 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate 102. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon or silicon carbide, a compound semiconductor substrate of silicon germanium, an SOI substrate, or the like can be used, or any of these substrates provided with a semiconductor element may be used as the substrate 102. In the case where a glass substrate is used as the substrate 102, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be fabricated.

Alternatively, a flexible substrate may be used as the substrate 102, and the transistor 100 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 102 and the transistor 100. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 102 and transferred onto another substrate. In such a case, the transistor 100 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

[Conductive Film]

The conductive film 104 functioning as a gate electrode and the conductive films 112 a and 112 b functioning as a source electrode and a drain electrode can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal elements as its component; an alloy including a combination of any of these metal elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b can be formed using an oxide conductor or an oxide semiconductor, such as an oxide including indium and tin (In—Sn oxide), an oxide including indium and tungsten (In—W oxide), an oxide including indium, tungsten, and zinc (In—W—Zn oxide), an oxide including indium and titanium (In—Ti oxide), an oxide including indium, titanium, and tin (In—Ti—Sn oxide), an oxide including indium and zinc (In—Zn oxide), an oxide including indium, tin, and silicon (In—Sn—Si oxide), or an oxide including indium, gallium, and zinc (In—Ga—Zn oxide).

Here, an oxide conductor is described. In this specification and the like, an oxide conductor may be referred to as OC. For example, oxygen vacancies are formed in an oxide semiconductor, and then hydrogen is added to the oxygen vacancies, so that a donor level is formed in the vicinity of the conduction band. This increases the conductivity of the oxide semiconductor; accordingly, the oxide semiconductor becomes a conductor. The oxide semiconductor having become a conductor can be referred to as an oxide conductor. Oxide semiconductors generally transmit visible light because of their large energy gap. Since an oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band, the influence of absorption due to the donor level is small in an oxide conductor, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films 104, 112 a, and 112 b. The use of a Cu—X alloy film results in lower manufacturing costs because the film can be processed by wet etching.

Among the above-mentioned metal elements, any one or more elements selected from titanium, tungsten, tantalum, and molybdenum are preferably included in the conductive films 112 a and 112 b. In particular, a tantalum nitride film is preferably used for the conductive films 112 a and 112 b. A tantalum nitride film has conductivity and a high barrier property against copper or hydrogen. Because a tantalum nitride film releases little hydrogen from itself, it can be favorably used as the conductive film in contact with the oxide semiconductor film 108 or the conductive film in the vicinity of the oxide semiconductor film 108.

The conductive films 112 a and 112 b can be formed by electroless plating. As a material that can be deposited by electroless plating, for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.

[Insulating Film Functioning as Gate Insulating Film]

As the insulating film 106 functioning as a gate insulating film of the transistor 100, an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that the insulating film 106 may have a layered structure of two or more layers.

The insulating film 106 that is in contact with the oxide semiconductor film 108 functioning as a channel region of the transistor 100 is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film 106 is an insulating film capable of releasing oxygen. In order to provide the oxygen-excess region in the insulating film 106, the insulating film 106 is formed in an oxygen atmosphere, or the deposited insulating film 106 is subjected to heat treatment in an oxygen atmosphere, for example.

In the case where hafnium oxide is used for the insulating film 106, the following effect is attained. Hafnium oxide has higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide, the thickness of the insulating film 106 can be made large as compared with the case where silicon oxide is used; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide having a crystal structure has a higher dielectric constant than hafnium oxide having an amorphous structure. Therefore, it is preferable to use hafnium oxide having a crystal structure, in order to provide a transistor with a low off-state current. Examples of the crystal structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to the above examples.

In this embodiment, a layered film of a silicon nitride film and a silicon oxide film is formed as the insulating film 106. The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the gate insulating film of the transistor 100, the thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor 100 and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor 100.

[Oxide Semiconductor Film]

The oxide semiconductor film 108 can be formed using the materials described above.

In the case where the oxide semiconductor film 108 includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In>M. The atomic ratio of metal elements in such a sputtering target is, for example, In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1.

In the case where the oxide semiconductor film 108 is formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film 108 having crystallinity. Note that the atomic ratio of metal elements in the formed oxide semiconductor film 108 varies from the above atomic ratios of metal elements of the sputtering targets in a range of ±40%. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the formed oxide semiconductor film 108 may be 4:2:3 or in the vicinity of 4:2:3.

The energy gap of the oxide semiconductor film 108 is 2 eV or more, preferably 2.5 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor 100 can be reduced.

The thickness of the oxide semiconductor film 108 is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm.

Furthermore, the oxide semiconductor film 108 may have a non-single-crystal structure. Examples of the non-single-crystal structure include a c-axis-aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, and an amorphous structure. Among the non-single-crystal structures, the amorphous structure has the highest density of defect states, whereas the CAAC-OS has the lowest density of defect states.

[Insulating Film 1 Functioning as Protective Insulating Film]

The insulating films 114 and 116 function as protective insulating films for the transistor 100. In addition, the insulating films 114 and 116 each have a function of supplying oxygen to the oxide semiconductor film 108. That is, the insulating films 114 and 116 contain oxygen. The insulating film 114 is an insulating film that allows oxygen to pass therethrough. Note that the insulating film 114 also functions as a film that relieves damage to the oxide semiconductor film 108 at the time of forming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in the insulating film 114 be small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film 114 is high, oxygen is bonded to the defects and the property of transmitting oxygen of the insulating film 114 is lowered.

Note that all oxygen entering the insulating film 114 from the outside does not move to the outside of the insulating film 114 and some oxygen remains in the insulating film 114. Furthermore, movement of oxygen occurs in the insulating film 114 in some cases in such a manner that oxygen enters the insulating film 114 and oxygen included in the insulating film 114 moves to the outside of the insulating film 114. When an oxide insulating film that can transmit oxygen is formed as the insulating film 114, oxygen released from the insulating film 116 provided over the insulating film 114 can be moved to the oxide semiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (E_(v) _(_) _(os)) and the energy of the conduction band minimum (E_(c) _(_) _(os)) of the oxide semiconductor film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in thermal desorption spectroscopy (TDS) analysis; the amount of released ammonia is typically greater than or equal to 1×10¹⁸ cm⁻³ and less than or equal to 5×10¹⁹ cm⁻³. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of a film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO₂ or NO, forms levels in the insulating film 114, for example. The level is positioned in the energy gap of the oxide semiconductor film 108. Therefore, when nitrogen oxide is diffused to the interface between the insulating film 114 and the oxide semiconductor film 108, an electron is in some cases trapped by the level on the insulating film 114 side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108; thus, the threshold voltage of the transistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film 114 reacts with ammonia included in the insulating film 116 in heat treatment, nitrogen oxide included in the insulating film 114 is reduced. Therefore, an electron is hardly trapped at the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108.

By using such an oxide insulating film, the insulating film 114 can reduce the shift in the threshold voltage of the transistor, which leads to a smaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film 114, by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than 350° C., a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 corresponds to the sum of the spin densities of signals attributed to nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is.

The concentration of nitrogen of the above oxide insulating film measured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at a substrate temperature higher than or equal to 220° C. and lower than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed.

The insulating film 116 is an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition. Part of oxygen is released from the above oxide insulating film by heating. The amount of oxygen released from the oxide insulating film in TDS is more than or equal to 1.0×10¹⁹ atoms/cm³, preferably more than or equal to 3.0×10²⁰ atoms/cm³. Note that the amount of released oxygen is the total amount of oxygen released by heat treatment in a temperature range of 50° C. to 650° C. or 50° C. to 550° C. in TDS. In addition, the amount of released oxygen is the total amount of released oxygen converted into oxygen atoms in TDS.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116 be small, and typically the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 116 is provided more apart from the oxide semiconductor film 108 than the insulating film 114 is; thus, the insulating film 116 may have higher density of defects than the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films 114 and 116 cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films 114 and 116 is shown by a dashed line. Although a two-layer structure of the insulating films 114 and 116 is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of only the insulating film 114 or a layered structure of three or more layers may be employed.

[Insulating Film 2 Functioning as Protective Insulating Film]

The insulating film 118 functions as a protective insulating film for the transistor 100.

The insulating film 118 includes one or both of hydrogen and nitrogen.

Alternatively, the insulating film 118 includes nitrogen and silicon. The insulating film 118 has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. The provision of the insulating film 118 makes it possible to prevent outward diffusion of oxygen from the oxide semiconductor film 108, outward diffusion of oxygen included in the insulating films 114 and 116, and entry of hydrogen, water, or the like into the oxide semiconductor film 108 from the outside.

A nitride insulating film, for example, can be used as the insulating film 118. The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like.

Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor film described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal chemical vapor deposition (CVD) method. A metal organic chemical vapor deposition (MOCVD) method or an atomic layer deposition (ALD) method may be employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film. In a thermal CVD method, a source gas is introduced into a chamber, the chamber is set at an atmospheric pressure or a reduced pressure, and a film is deposited on a substrate.

Furthermore, in an ALD method, a source gas is introduced into a chamber, the chamber is set at an atmospheric pressure or a reduced pressure, and a film is deposited on a substrate.

The variety of films such as the conductive films, the insulating films, and the oxide semiconductor film in this embodiment can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH₃)₃. The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formula of dimethylzinc is Zn(CH₃)₂. Without limitation to the above combination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium, and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by a deposition apparatus using ALD method, two kinds of gases, that is, ozone (03) as an oxidizer and a source gas that is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H₂O as an oxidizer and a source gas that is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ or dinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using a deposition apparatus using an ALD method, a first tungsten film is formed using a WF₆ gas and a B₂H₆ gas, and then a second tungsten film is formed using a WF₆ gas and an H₂ gas. Note that a SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus using an ALD method, an InO layer is formed using an In(CH₃)₃ gas and an O₃ gas, a GaO layer is formed using a Ga(CH₃)₃ gas and an O₃ gas, and then a ZnO layer is formed using a Zn(CH₃)₂ gas and an O₃ gas. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H₂O gas that is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Furthermore, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

<1-3. Structure Example 2 of Semiconductor Device>

Next, variations of the transistor 100 illustrated in in FIGS. 1A to 1C are described with reference to FIGS. 2A to 2C and FIGS. 3A to 3C.

FIG. 2A is a plan view of a transistor 100A that is a semiconductor device of one embodiment of the present invention. FIG. 2B is a cross-sectional view taken along a dashed dotted line X1-X2 in FIG. 2A, and FIG. 2C is a cross-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 2A.

Note that the transistor 100A illustrated in FIGS. 2A and 2B is what is called a channel-protective transistor. Thus, the semiconductor device of one embodiment of the present invention can have either the channel-etched structure or the channel-protective structure.

In the transistor 100A, the insulating films 114 and 116 have an opening 141 a and an opening 141 b. The oxide semiconductor film 108 is connected to the conductive films 112 a and 112 b through the openings 141 a and 141 b. Furthermore, the insulating film 118 is formed over the conductive films 112 a and 112 b. The insulating films 114 and 116 function as channel protective films. Note that the other components of the transistor 100A are similar to those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

FIG. 3A is a plan view of a transistor 100B that is a semiconductor device of one embodiment of the present invention. FIG. 3B is a cross-sectional view taken along a dashed dotted line X1-X2 in FIG. 3A, and FIG. 3C is a cross-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 3A.

The transistor 100B includes, the conductive film 104 over the substrate 102, the insulating film 106 over the substrate 102 and the conductive film 104, the oxide semiconductor film 108 over the insulating film 106, the conductive film 112 a over the oxide semiconductor film 108, the conductive film 112 b over the oxide semiconductor film 108, the insulating film 114 over the oxide semiconductor film 108, the conductive film 112 a, and the conductive film 112 b, the insulating film 116 over the insulating film 114, a conductive film 120 a over the insulating film 116, a conductive film 120 b over the insulating film 116, and the insulating film 118 over the insulating film 116 and the conductive films 120 a and 120 b.

The insulating films 114 and 116 have an opening 142 a. The insulating films 106, 114, and 116 have an opening 142 b. The conductive film 120 a is electrically connected to the conductive film 104 through the opening 142 b. Furthermore, the conductive film 120 b is electrically connected to the conductive film 112 b through the opening 142 a.

Note that in the transistor 100B, the insulating film 106 functions as a first gate insulating film of the transistor 100B, the insulating films 114 and 116 function as a second gate insulating film of the transistor 100B, and the insulating film 118 functions as a protective insulating film of the transistor 100B. In the transistor 100B, the conductive film 104 functions as a first gate electrode, the conductive film 112 a functions as a source electrode, and the conductive film 112 b functions as a drain electrode. In the transistor 100B, the conductive film 120 a functions as a second gate electrode, and the conductive film 120 b functions as a pixel electrode of a display device.

As illustrated in FIG. 3C, the conductive film 120 a is electrically connected to the conductive film 104 through the opening 142 b. Accordingly, the conductive film 104 and the conductive film 120 a are supplied with the same potential.

As illustrated in FIG. 3C, the oxide semiconductor film 108 is positioned so as to face the conductive film 104 and the conductive film 120 a, and is sandwiched between the two conductive films functioning as the gate electrodes. The length in the channel length direction and the length in the channel width direction of the conductive film 120 a are longer than the length in the channel length direction and the length in the channel width direction of the oxide semiconductor film 108, respectively. The whole oxide semiconductor film 108 is covered with the conductive film 120 a with the insulating films 114 and 116 positioned therebetween.

In other words, the conductive film 104 and the conductive film 120 a are connected through the opening provided in the insulating films 106, 114, and 116, and each include a region positioned outside an edge portion of the oxide semiconductor film 108.

With this structure, the oxide semiconductor film 108 included in the transistor 100B can be electrically surrounded by electric fields of the conductive films 104 and 120 a. A device structure of a transistor, like that of the transistor 100B, in which electric fields of a first gate electrode and a second gate electrode electrically surround an oxide semiconductor film where a channel region is formed can be referred to as a surrounded channel (s-channel) structure.

Since the transistor 100B has the s-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 104 functioning as a first gate electrode; therefore, the current drive capability of the transistor 100B can be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, the size of the transistor 100B can be reduced. In addition, since the transistor 100B is surrounded by the conductive film 104 functioning as the first gate electrode and the conductive film 120 a functioning as the second gate electrode, the mechanical strength of the transistor 100B can be increased.

Note that for the conductive films 120 a and 120 b, materials similar to those described as the materials of the above-described conductive films 104, 112 a, and 112 b can be used. In particular, oxide conductor films (OC) are preferable as the conductive films 120 a and 120 b.

The other components of the transistor 100B are similar to those of the transistor 100 described above and have similar effects.

The structures of the transistors of this embodiment can be freely combined with each other.

<1-4. Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of the transistor 100B that is a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6C, and FIGS. 7A to 7C.

FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6C, and FIGS. 7A to 7C are cross-sectional views illustrating a manufacturing method of the semiconductor device.

In each of FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6C, and FIGS. 7A to 7C, the left part is a cross-sectional view in the channel length direction, and the right part is a cross-sectional view in the channel width direction.

First, a conductive film is formed over the substrate 102 and processed through a lithography process and an etching process, whereby the conductive film 104 functioning as the first gate electrode is formed. Then, the insulating film 106 functioning as the first gate insulating film is formed over the conductive film 104 (see FIG. 4A).

In this embodiment, a glass substrate is used as the substrate 102, and as the conductive film 104 functioning as the first gate electrode, a 50-nm-thick titanium film and a 200-nm-thick copper film are each formed by a sputtering method. A 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film as the insulating film 106 are formed by a PECVD method.

Note that the above-described silicon nitride film has a three-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. An example of the three-layer structure is as follows.

For example, the first silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as a source gas to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and a power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film can each be formed at a substrate temperature of lower than or equal to 350° C.

When the silicon nitride film has the above-described three-layer structure, for example, in the case where a conductive film including copper is used as the conductive film 104, the following effect can be obtained.

The first silicon nitride film can inhibit diffusion of copper from the conductive film 104. The second silicon nitride film has a function of releasing hydrogen and can improve withstand voltage of the insulating film functioning as a gate insulating film.

The third silicon nitride film releases a small amount of hydrogen and can inhibit diffusion of hydrogen released from the second silicon nitride film.

Next, an oxide semiconductor film 108 i_1_0, an oxide semiconductor film 108 n_0, and an oxide semiconductor film 108 i_2_0 are formed over the insulating film 106 (see FIGS. 4B and 4C).

FIG. 4B is a schematic cross-sectional view illustrating the inside of a deposition apparatus when the oxide semiconductor film 108 i_1_0, the oxide semiconductor film 108 n_0, and the oxide semiconductor film 108 i_2_0 are formed over the insulating film 106. In FIG. 4B, a sputtering apparatus is used as the deposition apparatus, and a target 191 placed inside the sputtering apparatus and plasma 192 formed under the target 191 are schematically illustrated.

When the oxide semiconductor film 108 i_1_0 is formed, plasma discharge is performed in an atmosphere containing an oxygen gas. At this time, oxygen is added to the insulating film 106 over which the oxide semiconductor film 108 i_1_0 is to be formed. When the oxide semiconductor film 108 i_1_0 is formed, an inert gas (e.g., a helium gas, an argon gas, or a xenon gas) and the oxygen gas may be mixed.

The oxygen gas is mixed at least when the oxide semiconductor film 108 i_1_0 is formed. The proportion of the oxygen gas in a deposition gas for forming the oxide semiconductor film 108 i_1_0 (percentage of oxygen flow rate) is higher than 0% and lower than or equal to 100%, preferably higher than or equal to 10% and lower than or equal to 100%, further preferably higher than or equal to 30% and lower than or equal to 100%.

In FIG. 4B, oxygen or excess oxygen added to the insulating film 106 is schematically shown by arrows of broken lines.

Furthermore, either or both of the substrate temperature and the percentage of oxygen flow rate in forming the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0 are preferably higher than those in forming the oxide semiconductor film 108 n_0.

Specifically, the conditions for forming the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0 are set as follows: the substrate temperature is higher than or equal to 150° C. and lower than or equal to 300° C., preferably higher than or equal to 160° C. and lower than or equal to 200° C., and the percentage of oxygen flow rate is higher than or equal to 30% and lower than or equal to 100%. Furthermore, the conditions for forming the oxide semiconductor film 108 n_0 are set as follows: the substrate temperature is higher than or equal to room temperature and lower than 150° C., preferably higher than or equal to 100° C. and lower than or equal to 140° C., and the percentage of oxygen flow rate is higher than 0% and lower than 30%.

Under the above-described conditions, the oxide semiconductor films having different carrier densities can be stacked. Note that it is more favorable to successively form the oxide semiconductor film 108 i_1_0, the oxide semiconductor film 108 n_0, and the oxide semiconductor film 108 i_2_0 in vacuum because impurities can be prevented from being caught at the interfaces.

When the oxide semiconductor film 108 is formed while being heated, the crystallinity of the oxide semiconductor film 108 can be increased. However, in the case where a large-sized glass substrate (e.g., the 6th generation to the 10th generation) is used as the substrate 102 and the oxide semiconductor film 108 is formed at a substrate temperature higher than or equal to 200° C. and lower than or equal to 300° C., the substrate 102 might be changed in shape (distorted or warped). In the case where a large-sized glass substrate is used, the change in the shape of the glass substrate can be suppressed by forming the oxide semiconductor film 108 at a substrate temperature higher than or equal to 100° C. and lower than 200° C.

In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used as a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be minimized.

In the case where the oxide semiconductor film is deposited by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the oxide semiconductor film, as much as possible. In particular, the partial pressure of gas molecules corresponding to H₂O (gas molecules corresponding to m/z=18) in the chamber in the standby mode of the sputtering apparatus is preferably lower than or equal to 1×10⁻⁴ Pa, further preferably 5×10⁻⁵ Pa.

In this embodiment, the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0 are formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio). The substrate temperature during the formation of the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0 is 170° C., and oxygen gas at a flow rate of 60 sccm and argon gas at a flow rate of 140 sccm are used as a deposition gas (percentage of oxygen flow rate: 30%).

In addition, the oxide semiconductor film 108 n_0 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio). The substrate temperature during the formation of the oxide semiconductor film 108 n_0 is 130° C., and oxygen gas at a flow rate of 20 sccm and argon gas at a flow rate of 180 sccm are used as a deposition gas (percentage of oxygen flow rate: 10%).

Note that although the stacked structure of the oxide semiconductor films having different carrier densities was formed by changing the substrate temperature and the percentage of oxygen flow rate among the oxide semiconductor films 108 i_1_0, 108 n_0, and 108 i_2_0 in the above-described example, the method for forming the structure is not limited to this example. For example, an impurity element may be added in formation of the oxide semiconductor film 108 n_0 to make the carrier density of the oxide semiconductor film 108 n_0 different from those of the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0. Examples of the impurity element include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, and a rare gas element.

Among the above-described elements, nitrogen is particularly preferable as the impurity element added to the oxide semiconductor film 108 n_0. For example, nitrogen can be added to the oxide semiconductor film 108 n_0 by using argon gas and nitrogen gas as a deposition gas or using argon gas and dinitrogen monoxide as a deposition gas in forming the oxide semiconductor film 108 n_0.

In the case where an impurity element is used to form the oxide semiconductor film 108 n_0, it is favorable to independently provide a chamber for forming the oxide semiconductor film 108 n_0 in order to prevent the impurity element from entering a film into which the impurity element is preferably not added, e.g., the oxide semiconductor film 108 i_1_0 and the oxide semiconductor film 108 i_2_0.

After the oxide semiconductor film 108 n_0 is formed, an impurity element may be added to the oxide semiconductor film 108 n_0. As a method for adding an impurity element after formation of the oxide semiconductor film 108 n_0, doping treatment or plasma treatment can be used, for example.

Next, the oxide semiconductor film 108 i_1_0, the oxide semiconductor film 108 n_0, and the oxide semiconductor film 108 i_2_0 are processed into desired shapes, so that the island-shaped oxide semiconductor film 108 i_1, the island-shaped oxide semiconductor film 108 n, and the island-shaped oxide semiconductor film 108 i_2 are formed. In this embodiment, the oxide semiconductor film 108 i_1, the oxide semiconductor film 108 n, and the oxide semiconductor film 108 i_2 constitute the oxide semiconductor film 108 (see FIG. 5A).

Heat treatment (hereinafter referred to as first heat treatment) is preferably performed after the oxide semiconductor film 108 is formed. By the first heat treatment, water, hydrogen, or the like contained in the oxide semiconductor film 108 can be reduced. The heat treatment for the purpose of reducing hydrogen, water, and the like may be performed before the oxide semiconductor film 108 is processed into an island shape. Note that the first heat treatment is one kind of treatment for increasing the purity of the oxide semiconductor film.

The first heat treatment can be performed at a temperature of, for example, higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 200° C. and lower than or equal to 450° C., further preferably higher than or equal to 250° C. and lower than or equal to 350° C.

Moreover, an electric furnace, an RTA apparatus, or the like can be used for the first heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened. The first heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air with a water content of 20 ppm or less, preferably 1 ppm or less, further preferably ppb or less), or a rare gas (e.g., argon, helium). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. Furthermore, after heat treatment is performed under a nitrogen atmosphere or a rare gas atmosphere, heat treatment may be additionally performed in an oxygen atmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water, and the like can be released from the oxide semiconductor film and oxygen can be supplied to the oxide semiconductor film at the same time. Consequently, the number of oxygen vacancies in the oxide semiconductor film can be reduced.

Next, a conductive film 112 is formed over the insulating film 106 and the oxide semiconductor film 108 (see FIG. 5B).

In this embodiment, as the conductive film 112, a 30-nm-thick titanium film, a 200-nm-thick copper film, and a 10-nm-thick titanium film are formed in this order by a sputtering method.

Next, the conductive film 112 is processed into a desired shape, so that the island-shaped conductive film 112 a and the island-shaped conductive film 112 b are formed (see FIG. 5C).

In this embodiment, the conductive film 112 is processed with a wet etching apparatus. Note that the method for processing the conductive film 112 is not limited to the above-described method, and a dry etching apparatus may be used, for example.

After the conductive films 112 a and 112 b are formed, a surface (on the back channel side) of the oxide semiconductor film 108 (specifically, the oxide semiconductor film 108 i_2) may be cleaned. The cleaning may be performed, for example, using a chemical solution such as a phosphoric acid. The cleaning using a chemical solution such as a phosphoric acid can remove impurities (e.g., an element included in the conductive films 112 a and 112 b) attached to the surface of the oxide semiconductor film 108 i_2. Note that the cleaning is not necessarily performed; in some cases, the cleaning is not performed.

In the step of forming the conductive films 112 a and 112 b and/or the cleaning step, the thickness of a region of the oxide semiconductor film 108 which is not covered with the conductive films 112 a and 112 b might be reduced.

Next, the insulating film 114 and the insulating film 116 are formed over the oxide semiconductor film 108 and the conductive films 112 a and 112 b (see FIG. 6A).

Note that after the insulating film 114 is formed, the insulating film 116 is preferably formed successively without exposure to the air. When the insulating film 116 is formed successively after the formation of the insulating film 114 without exposure to the air while at least one of the flow rate of a source gas, the pressure, high-frequency power, and the substrate temperature is adjusted, the concentration of impurities attributed to the atmospheric component at the interface between the insulating films 114 and 116 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film can be formed by a PECVD method. In that case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include dinitrogen monoxide and nitrogen dioxide. The flow rate of the oxidizing gas is more than or equal to 20 times and less than or equal to 500 times, preferably more than or equal to 40 times and less than or equal to 100 times, that of the deposition gas.

In this embodiment, a silicon oxynitride film is formed as the insulating film 114 by a PECVD method under the conditions where the substrate 102 is held at a temperature of 220° C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are used as a source gas, the pressure in the treatment chamber is Pa, and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the power density) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in the treatment chamber of the PECVD apparatus that is vacuum-evacuated is held at a temperature of higher than or equal to 180° C. and lower than or equal to 350° C., the pressure in the treatment chamber is higher than or equal to 100 Pa and lower than or equal to 250 Pa, preferably higher than or equal to 100 Pa and lower than or equal to 200 Pa, with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², preferably greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 116, the high-frequency power having the above power density is supplied to the reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the insulating film 116 becomes higher than that in the stoichiometric composition. In the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in a later step. Thus, it is possible to form an oxide insulating film which contains more oxygen than that in the stoichiometric composition and from which part of oxygen is released by heating.

Note that the insulating film 114 functions as a protective film for the oxide semiconductor film 108 in the step of forming the insulating film 116. Therefore, the insulating film 116 can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, when the flow rate of the deposition gas containing silicon with respect to the oxidizing gas is increased, the amount of defects in the insulating film 116 can be reduced. Typically, it is possible to form an oxide insulating film in which the amount of defects is small, i.e. the spin density of a signal which appears at g=2.001 due to a dangling bond of silicon, is lower than 6×10¹? spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, further preferably lower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, the reliability of the transistor 100 can be improved.

Heat treatment (hereinafter referred to as second heat treatment) is preferably performed after the insulating films 114 and 116 are formed. The second heat treatment can reduce nitrogen oxide included in the insulating films 114 and 116. By the second heat treatment, part of oxygen contained in the insulating films 114 and 116 can be transferred to the oxide semiconductor film 108, so that the amount of oxygen vacancies included in the oxide semiconductor film 108 can be reduced.

The temperature of the second heat treatment is typically lower than 400° C., preferably lower than 375° C., further preferably higher than or equal to 150° C. and lower than or equal to 350° C. The second heat treatment may be performed in an atmosphere of nitrogen, oxygen, ultra-dry air (air with a water content of less than or equal to 20 ppm, preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb), or a rare gas (e.g., argon, helium). Note that it is preferable that hydrogen, water, and the like be not contained in the nitrogen, oxygen, ultra-dry air, or rare gas. An electric furnace, RTA, or the like can be used for the heat treatment.

Next, the openings 142 a and 142 b are formed in desired regions in the insulating films 114 and 116 (see FIG. 6B).

In this embodiment, the openings 142 a and 142 b are formed with a dry etching apparatus. Note that the opening 142 a reaches the conductive film 112 b, and the opening 142 b reaches the conductive film 104.

Next, a conductive film 120 is formed over the insulating film 116 (see FIG. 6C and FIG. 7A).

FIG. 6C is a schematic cross-sectional view illustrating the inside of a deposition apparatus when the conductive film 120 is formed over the insulating film 116. In FIG. 6C, a sputtering apparatus is used as the deposition apparatus, and a target 193 placed inside the sputtering apparatus and plasma 194 formed under the target 193 are schematically illustrated.

When the conductive film 120 is formed, plasma discharge is performed in an atmosphere containing an oxygen gas. At this time, oxygen is added to the insulating film 116 over which the conductive film 120 is to be formed. When the conductive film 120 is formed, an inert gas (e.g., a helium gas, an argon gas, or a xenon gas) and the oxygen gas may be mixed.

The oxygen gas is mixed at least when the conductive film 120 is formed.

The proportion of the oxygen gas in a deposition gas for forming the conductive film 120 is higher than 0% and lower than or equal to 100%, preferably higher than or equal to 10% and lower than or equal to 100%, further preferably higher than or equal to 30% and lower than or equal to 100%.

In FIG. 6C, oxygen or excess oxygen added to the insulating film 116 is schematically shown by arrows of broken lines.

In this embodiment, the conductive film 120 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio).

Note that although oxygen is added to the insulating film 116 when the conductive film 120 is formed in this embodiment, the method for adding oxygen is not limited to this example. For example, oxygen may be further added to the insulating film 116 after the conductive film 120 is formed.

As the method for adding oxygen to the insulating film 116, an ITSO film with a thickness of 5 nm may be formed using a target of an oxide including indium, tin, and silicon (an In—Sn—Si oxide, also referred to as ITSO) (In₂O₃:SnO₂:SiO₂=85:10:5 in wt %), for example. In that case, the thickness of the ITSO film is preferably greater than or equal to 1 nm and less than or equal to 20 nm or greater than or equal to 2 nm and less than or equal to 10 nm, in which case oxygen is favorably transmitted and release of oxygen can be inhibited. Then, oxygen is added to the insulating film 116 through the ITSO film. Oxygen can be added by, for example, ion doping, ion implantation, or plasma treatment. By application of a bias voltage to the substrate side when oxygen is added, oxygen can be effectively added to the insulating film 116. An ashing apparatus is used, for example, and the power density of the bias voltage applied to the substrate side of the ashing apparatus can be greater than or equal to 1 W/cm² and less than or equal to 5 W/cm². The substrate temperature during addition of oxygen is higher than or equal to room temperature and lower than or equal to 300° C., preferably higher than or equal to 100° C. and lower than or equal to 250° C., whereby oxygen can be added efficiently to the insulating film 116.

Next, the conductive film 120 is processed into a desired shape, so that the island-shaped conductive films 120 a and 120 b are formed (see FIG. 7B).

In this embodiment, the conductive film 120 is processed with a wet etching apparatus.

Next, the insulating film 118 is formed over the insulating film 116 and the conductive films 120 a and 120 b (see FIG. 7C).

The insulating film 118 includes either or both of hydrogen and nitrogen. As the insulating film 118, a silicon nitride film is preferably used, for example. The insulating film 118 can be formed by a sputtering method or a PECVD method, for example. In the case where the insulating film 118 is formed by a PECVD method, for example, the substrate temperature is lower than 400° C., preferably lower than 375° C., and further preferably higher than or equal to 180° C. and lower than or equal to 350° C. The substrate temperature at which the insulating film 118 is formed is preferably within the above range because a dense film can be formed. Furthermore, when the substrate temperature at which the insulating film 118 is formed is within the above range, oxygen or excess oxygen in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108.

In the case where a silicon nitride film is formed by a PECVD method as the insulating film 118, a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. A small amount of ammonia compared with the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species cleave a bond between silicon and hydrogen which are included in a deposition gas including silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen are few, can be formed. If the amount of ammonia with respect to nitrogen is large, decomposition of a deposition gas including silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, the flow rate of nitrogen is set to be preferably 5 times or more and 50 times or less, further preferably 10 times or more and 50 times or less the flow rate of ammonia.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thick silicon nitride film is formed as the insulating film 118 using silane, nitrogen, and ammonia as a source gas. The flow rate of silane is 50 sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate of ammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a 27.12 MHz high-frequency power source. Note that the PECVD apparatus is a parallel-plate PECVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 1.7×10⁻¹ W/cm².

In the case where the conductive films 120 a and 120 b are formed using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio), either or both of hydrogen and nitrogen included in the insulating film 118 might enter the conductive films 120 a and 120 b. In this case, either or both of hydrogen and nitrogen might be bonded to oxygen vacancies in the conductive films 120 a and 120 b to cause a reduction in the resistance of the conductive films 120 a and 120 b.

After the insulating film 118 is formed, heat treatment similar to the first heat treatment or the second heat treatment (hereinafter referred to as third heat treatment) may be performed.

By the third heat treatment, oxygen included in the insulating film 116 moves into the oxide semiconductor film 108 to fill the oxygen vacancies in the oxide semiconductor film 108.

Through the above process, the transistor 100B illustrated in FIGS. 3A to 3C can be manufactured.

The transistor 100 illustrated in FIGS. 1A to 1C can be manufactured by forming the insulating film 118 after the step of FIG. 6A. The transistor 100A illustrated in FIGS. 2A to 2C can be manufactured by changing the formation order of the conductive films 112 a and 112 b and the insulating films 114 and 116 and, in addition, adding a step for forming the openings 141 a and 141 b in the insulating films 114 and 116.

Note that the structure and method described in this embodiment can be used in appropriate combination with the structure and method described in any of the other embodiments.

Embodiment 2

In this embodiment, a semiconductor device of one embodiment of the present invention and a manufacturing method of the semiconductor device are described with reference to FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A and 10B, FIGS. 11A and 11B, FIGS. 12A to 12D, FIGS. 13A to 13C, and FIGS. 14A to 14C.

<2-1. Structural Example 1 of Semiconductor Device>

FIG. 8A is a plan view of a transistor 200 that is a semiconductor device of one embodiment of the present invention. FIG. 8B is a cross-sectional view taken along a dashed dotted line X1-X2 in FIG. 8A, and FIG. 8C is a cross-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 8A. Note that in FIG. 8A, some components of the transistor 200 (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. Furthermore, the direction of the dashed dotted line X1-X2 may be referred to as a channel length direction, and the direction of the dashed dotted line Y1-Y2 may be referred to as a channel width direction. As in FIG. 8A, some components are not illustrated in some cases in plan views of transistors described below.

The transistor 200 illustrated in FIGS. 8A to 8C is what is called a top-gate transistor.

The transistor 200 includes an insulating film 204 over a substrate 202, an oxide semiconductor film 208 over the insulating film 204, an insulating film 210 over the oxide semiconductor film 208, a conductive film 212 over the insulating film 210, and an insulating film 216 over the insulating film 204, the oxide semiconductor film 208, and the conductive film 212.

The oxide semiconductor film 208 preferably includes In, M (M is Al, Ga, Y, or Sn), and Zn.

The oxide semiconductor film 208 includes a first region 208 n_1 overlapping with the conductive film 212 and being in contact with the insulating film 204, a second region 208 i overlapping with the conductive film 212 and being in contact with the insulating film 210, and a third region 208 n_2 being in contact with the insulating film 216. The first region 208 n_1 includes a region having a higher carrier density than the second region 208 i, and the third region 208 n_2 includes a region having a higher carrier density than the second region 208 i. In other words, the oxide semiconductor film 208 of one embodiment of the present invention includes three regions having different carrier densities.

Note that the carrier density of the oxide semiconductor film 208 is preferably higher than or equal to 1×10⁵ cm⁻³ and lower than 1×10¹⁸ cm⁻³, further preferably higher than or equal to 1×10⁷ cm⁻³ and lower than or equal to 1×10¹⁷ cm⁻³, still further preferably higher than or equal to 1×10⁹ cm⁻³ and lower than or equal to 5×10¹⁶ cm⁻³, yet further preferably higher than or equal to 1×10¹⁰ cm⁻³ and lower than or equal to 1×10¹⁶ cm⁻³, and yet still preferably higher than or equal to 1×10¹¹ cm⁻³ and lower than or equal to 1×10¹⁵ cm⁻³.

For example, the oxide semiconductor film 208 is formed to have a two-layer structure of a first oxide semiconductor film and a second oxide semiconductor film over the first oxide semiconductor film. By making the first oxide semiconductor film have a carrier density higher than the second oxide semiconductor film, the oxide semiconductor film including regions with different carrier densities as described above can be formed.

The amount of oxygen vacancies or the impurity concentration in the first oxide semiconductor film is slightly higher than that of the second oxide semiconductor film.

To increase the carrier density of the first oxide semiconductor film, an element that forms oxygen vacancies may be added into the first oxide semiconductor film so that hydrogen or the like can be bonded to the oxygen vacancies. Typical examples of the element that forms oxygen vacancies include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, and a rare gas element. Typical examples of the rare gas element are helium, neon, argon, krypton, and xenon. Note that among the above-mentioned elements, nitrogen is particularly preferable as the element that forms oxygen vacancies in the oxide semiconductor film.

For example, by using argon gas and dinitrogen monoxide gas as a deposition gas in forming the first oxide semiconductor film, nitrogen can be contained in the first oxide semiconductor film. In this case, the first oxide semiconductor film includes a region having a higher nitrogen concentration than the second oxide semiconductor film.

Accordingly, the first oxide semiconductor film has a higher carrier density and is of slightly n-type. An oxide semiconductor film having a higher carrier density is described as “slightly-n”, in some cases.

For example, in the case where the voltage applied to the gate of the transistor (V_(g)) is higher than 0 V and lower than or equal to 30 V, the carrier density of the first oxide semiconductor film is preferably higher than 1×1016 cm⁻³ and lower than 1×10¹⁸ cm⁻³, further preferably higher than 1×10¹⁶ cm⁻³ and lower than or equal to 1×10¹⁷ cm⁻³.

In the case where the carrier density of the first oxide semiconductor film is increased, the crystallinity of the first oxide semiconductor film is lower than that of the second oxide semiconductor film, in some cases. In this case, the oxide semiconductor film 208 has a layered structure including a low-crystallinity oxide semiconductor film and a high-crystallinity oxide semiconductor film. Since the crystallinity of an oxide semiconductor film has a correlation to the film density of the oxide semiconductor film, the oxide semiconductor film having higher crystallinity has a higher film density. Thus, the oxide semiconductor film 208 has a layered structure including an oxide semiconductor film having a low film density and an oxide semiconductor film having a high film density.

Note that the crystallinity of the oxide semiconductor film 208 can be determined by analysis by X-ray diffraction (XRD) or with a transmission electron microscope (TEM). Furthermore, the film density of the oxide semiconductor film 208 can be measured with an X-ray reflectometer (XRR), for example.

Furthermore, the third region 208 n_2 is in contact with the insulating film 216. The insulating film 216 includes nitrogen or hydrogen. Thus, nitrogen or hydrogen included in the insulating film 216 is added to the third region 208 n_2. The carrier density of the third region 208 n_2 is increased by the addition of nitrogen or hydrogen from the insulating film 216.

The transistor 200 may further include an insulating film 218 over the insulating film 216, a conductive film 220 a electrically connected to the third region 208 n_2 through an opening 241 a provided in the insulating films 216 and 218, and a conductive film 220 b electrically connected to the third region 208 n_2 through an opening 241 b provided in the insulating films 216 and 218.

In this specification and the like, the insulating film 204 may be referred to as a first insulating film, the insulating film 210 may be referred to as a second insulating film, the insulating film 216 may be referred to as a third insulating film, and the insulating film 218 may be referred to as a fourth insulating film. The conductive film 212 functions as a gate electrode, the conductive film 220 a functions as a source electrode, and the conductive film 220 b functions as a drain electrode.

The insulating film 210 functions as a gate insulating film. The insulating film 210 includes an oxygen-excess region. Since the insulating film 210 includes the oxygen-excess region, excess oxygen can be supplied to the second region 208 i included in the oxide semiconductor film 208. Therefore, oxygen vacancies that might be formed in the channel region 208 i can be filled with excess oxygen; as a result, a highly reliable semiconductor device can be provided.

To supply excess oxygen to the oxide semiconductor film 208, excess oxygen may be supplied to the insulating film 204 that is formed under the oxide semiconductor film 208. However, in that case, excess oxygen contained in the insulating film 204 might also be supplied to the first region 208 n_1 and the third region 208 n_2 included in the oxide semiconductor film 208. When excess oxygen is supplied to the first region 208 n_1 and the third region 208 n_2, the resistance of the first region 208 n_1 and the third region 208 n_2 might be increased.

In contrast, in the structure in which the insulating film 210 formed over the oxide semiconductor film 208 contains excess oxygen, excess oxygen can be selectively supplied only to the second region 208 i. Alternatively, the carrier density of the third region 208 n_2 can be selectively increased after excess oxygen is supplied to the first region 208 n_1, the second region 208 i, and the third region 208 n_2; thus, an increase in the resistance of the third region 208 n_2 can be prevented.

The oxide semiconductor film 208 preferably includes a region in which the atomic proportion of In is larger than the atomic proportion of M. When the oxide semiconductor film 208 includes a region in which the atomic proportion of In is larger than the atomic proportion of M, the transistor 200 can have high field-effect mobility. Specifically, the field-effect mobility of the transistor 200 can exceed 10 cm²/Vs, preferably exceed 30 cm²/Vs.

For example, the use of the transistor with high field-effect mobility in a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame.

When the oxide semiconductor film 208 i includes a region in which the atomic proportion of In is larger than the atomic proportion of M, on the other hand, electric characteristics, e.g., the threshold, of the transistor 200 may vary in the gate bias temperature stress (GBT) test.

However, in the semiconductor device of one embodiment of the present invention, the oxide semiconductor film 208 includes the first region 208 n_1, the second region 208 i, and the third region 208 n_2, and the first region 208 n_1 includes a region having a higher carrier density than second region 208 i.

The increased carrier density in the first region 208 n_1 included in the oxide semiconductor film 208 can increase the Fermi level relative to the conduction band of the first region 208 n_1. This lowers the conduction band minimum of the first region 208 n_1, so that the energy difference between the conduction band minimum of the first region 208 n_1 and the trap level, which might be formed in a gate insulating film (here, the insulating film 210), can be increased. The increase of the energy difference can reduce trap of charges in the gate insulating film and reduce variation in the threshold voltage of the transistor in the GBT test, in some cases.

Impurities such as hydrogen or moisture entering the oxide semiconductor film 208 adversely affect the transistor characteristics and therefore cause a problem. Therefore, it is preferable that the amount of impurities such as hydrogen or moisture in the oxide semiconductor film 208, particularly in the second region 208 i, be as small as possible. Furthermore, oxygen vacancies formed in the oxide semiconductor film 208 adversely affect the transistor characteristics and therefore cause a problem. For example, oxygen vacancies formed in the oxide semiconductor film 208 are bonded to hydrogen to serve as a carrier supply source. The carrier supply source generated in the oxide semiconductor film 208 causes a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor 200 including the oxide semiconductor film 208. Therefore, it is preferable that the amount of oxygen vacancies in the oxide semiconductor film 208, particularly in the second region 208 i, be as small as possible.

Oxygen vacancies that might be formed in the second region 208 i can be filled with excess oxygen supplied from the insulating film 210. Thus, the second region 208 i included in the oxide semiconductor film 208 is an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low. Note that the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

As described above, the semiconductor device of one embodiment of the present invention has a two-layer structure of oxide semiconductor films having different carrier densities, which is formed by controlling the amount of oxygen vacancies or the impurity concentration. By increasing the carrier density of the lower-layer oxide semiconductor film and decreasing the carrier density of the upper-layer oxide semiconductor film, both the field-effect mobility and reliability of the transistor can be improved. Furthermore, in the semiconductor device of one embodiment of the present invention, the carrier density of the oxide semiconductor film in the regions that are in contact with the source electrode or the drain electrode is further reduced. This can decrease the contact resistance between the oxide semiconductor film and the conductive film serving as the source electrode or the drain electrode. Thus, a semiconductor device with excellent electrical characteristics can be provided.

<2-2. Components of Semiconductor Device>

Next, components of the semiconductor device of this embodiment are described in detail.

[Substrate]

There is no particular limitation on the property of a material and the like of the substrate 202 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate 202. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon or silicon carbide, a compound semiconductor substrate of silicon germanium, an SOI substrate, or the like can be used, or any of these substrates provided with a semiconductor element may be used as the substrate 202. In the case where a glass substrate is used as the substrate 202, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be fabricated.

Alternatively, a flexible substrate may be used as the substrate 202, and the transistor 200 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 202 and the transistor 200. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 202 and transferred onto another substrate. In such a case, the transistor 200 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

[First Insulating Film]

The insulating film 204 can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like as appropriate. For example, the insulating film 204 can be formed to have a single-layer structure or stacked-layer structure of an oxide insulating film and/or a nitride insulating film. To improve the properties of the interface with the oxide semiconductor film 208, at least a region of the insulating film 204 which is in contact with the oxide semiconductor film 208 is preferably formed using an oxide insulating film. When the insulating film 204 is formed using an oxide insulating film from which oxygen is released by heating, oxygen contained in the insulating film 204 can be moved to the oxide semiconductor film 208 by heat treatment.

The thickness of the insulating film 204 can be greater than or equal to 50 nm, greater than or equal to 100 nm and less than or equal to 3000 nm, or greater than or equal to 200 nm and less than or equal to 1000 nm. By increasing the thickness of the insulating film 204, the amount of oxygen released from the insulating film 204 can be increased, and interface states at the interface between the insulating film 204 and the oxide semiconductor film 208 and oxygen vacancies included in the oxide semiconductor film 208 can be reduced.

For example, the insulating film 204 can be formed to have a single-layer structure or stacked-layer structure of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, a Ga—Zn oxide, or the like. In this embodiment, the insulating film 204 has a stacked-layer structure of a silicon nitride film and a silicon oxynitride film. With the insulating film 204 having such a layered structure including a silicon nitride film as a lower layer and a silicon oxynitride film as an upper layer, oxygen can be efficiently introduced into the oxide semiconductor film 208.

[Conductive Film]

The conductive film 212 functioning as a gate electrode and the conductive films 220 a and 220 b functioning as a source electrode and a drain electrode can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal elements as its component; an alloy including a combination of any of these metal elements; or the like.

Furthermore, the conductive films 212, 220 a, and 220 b can be formed using an oxide conductor or an oxide semiconductor, such as an oxide including indium and tin (In—Sn oxide), an oxide including indium and tungsten (In—W oxide), an oxide including indium, tungsten, and zinc (In—W—Zn oxide), an oxide including indium and titanium (In—Ti oxide), an oxide including indium, titanium, and tin (In—Ti—Sn oxide), an oxide including indium and zinc (In—Zn oxide), an oxide including indium, tin, and silicon (In—Sn—Si oxide), or an oxide including indium, gallium, and zinc (In—Ga—Zn oxide).

Here, an oxide conductor is described. In this specification and the like, an oxide conductor may be referred to as OC. For example, oxygen vacancies are formed in an oxide semiconductor, and then hydrogen is added to the oxygen vacancies, so that a donor level is formed in the vicinity of the conduction band. This increases the conductivity of the oxide semiconductor; accordingly, the oxide semiconductor becomes a conductor. The oxide semiconductor having become a conductor can be referred to as an oxide conductor. Oxide semiconductors generally transmit visible light because of their large energy gap. Since an oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band, the influence of absorption due to the donor level is small in an oxide conductor, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

In particular, the above-described oxide conductor is favorably used as the conductive film 212 because excess oxygen can be added to the insulating film 210.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films 212, 220 a, and 220 b. The use of a Cu—X alloy film results in lower manufacturing costs because the film can be processed by wet etching.

Among the above-mentioned metal elements, any one or more elements selected from titanium, tungsten, tantalum, and molybdenum are preferably included in the conductive films 212, 220 a, and 220 b. In particular, a tantalum nitride film is preferably used for the conductive films 212, 220 a, and 220 b. A tantalum nitride film has conductivity and a high barrier property against copper or hydrogen. Because a tantalum nitride film releases little hydrogen from itself, it can be favorably used as the conductive film in contact with the oxide semiconductor film 208 or the conductive film in the vicinity of the oxide semiconductor film 208.

The conductive films 212, 220 a, and 220 b can be formed by electroless plating. As a material that can be deposited by electroless plating, for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.

[Second Insulating Film]

As the insulating film 210 functioning as a gate insulating film of the transistor 200, an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that the insulating film 210 may have a layered structure of two or more layers.

The insulating film 210 that is in contact with the oxide semiconductor film 208 functioning as a channel region of the transistor 200 is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film 210 is an insulating film capable of releasing oxygen. In order to provide the oxygen-excess region in the insulating film 210, the insulating film 210 is formed in an oxygen atmosphere, or the deposited insulating film 210 is subjected to heat treatment in an oxygen atmosphere, for example.

In the case where hafnium oxide is used for the insulating film 210, the following effect is attained. Hafnium oxide has higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide, the thickness of the insulating film 210 can be made large as compared with the case where silicon oxide is used; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide having a crystal structure has a higher dielectric constant than hafnium oxide having an amorphous structure. Therefore, it is preferable to use hafnium oxide having a crystal structure, in order to provide a transistor with a low off-state current. Examples of the crystal structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to the above examples.

It is preferable that the insulating film 210 have few defects and typically have as few signals observed by electron spin resonance (ESR) spectroscopy as possible. Examples of the signals include a signal due to an E′ center observed at a g-factor of 2.001. Note that the E′ center is due to the dangling bond of silicon. As the insulating film 210, a silicon oxide film or a silicon oxynitride film whose spin density of a signal due to the E′ center is lower than or equal to 3×10¹⁷ spins/cm³ and preferably lower than or equal to 5×10¹⁶ spins/cm³ may be used.

In addition to the above-described signal, a signal due to nitrogen dioxide (NO₂) might be observed in the insulating film 210. The signal is divided into three signals according to the N nuclear spin; a first signal, a second signal, and a third signal. The first signal is observed at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039. The second signal is observed at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003. The third signal is observed at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966.

It is suitable to use an insulating film whose spin density of a signal due to nitrogen dioxide (NO₂) is higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³ as the insulating film 210, for example.

Note that a nitrogen oxide (NO_(x)) such as nitrogen dioxide (NO₂) forms a state in the insulating film 210. The state is positioned in the energy gap of the oxide semiconductor film 208. Thus, when nitrogen oxide (NO_(x)) is diffused to the interface between the insulating film 210 and the oxide semiconductor film 208, an electron might be trapped by the state on the insulating film 210 side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film 210 and the oxide semiconductor film 208, leading to a positive shift of the threshold voltage of the transistor. Accordingly, the use of a film with a low nitrogen oxide content as the insulating film 210 can reduce a shift of the threshold voltage of the transistor.

As an insulating film that releases a small amount of nitrogen oxide (NO_(x)), for example, a silicon oxynitride film can be used. The silicon oxynitride film releases more ammonia than nitrogen oxide (NO_(x)) in thermal desorption spectroscopy (TDS); the typical released amount of ammonia is greater than or equal to 1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note that the released amount of ammonia is the total amount of ammonia released by heat treatment in a range of 50° C. to 650° C. or 50° C. to 550° C. in TDS.

Since nitrogen oxide (NO_(x)) reacts with ammonia and oxygen in heat treatment, the use of an insulating film that releases a large amount of ammonia reduces nitrogen oxide (NO_(x)).

Note that in the case where the insulating film 210 is analyzed by SIMS, nitrogen concentration in the film is preferably lower than or equal to 6×10²⁰ atoms/cm³.

[Oxide Semiconductor Film]

The oxide semiconductor film 208 can be formed using the materials described above.

In the case where the oxide semiconductor film 208 includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In>M. The atomic ratio of metal elements in such a sputtering target is, for example, In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1.

In the case where the oxide semiconductor film 208 is formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film 208 having crystallinity. Note that the atomic ratio of metal elements in the formed oxide semiconductor film 208 varies from the above atomic ratios of metal elements of the sputtering targets in a range of ±40%. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the formed oxide semiconductor film 208 may be 4:2:3 or in the vicinity of 4:2:3.

The energy gap of the oxide semiconductor film 208 is 2 eV or more, preferably 2.5 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor 200 can be reduced.

The thickness of the oxide semiconductor film 208 is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm.

Furthermore, the oxide semiconductor film 208 may have a non-single-crystal structure. Examples of the non-single-crystal structure include a c-axis-aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, and an amorphous structure. Among the non-single-crystal structures, the amorphous structure has the highest density of defect states, whereas the CAAC-OS has the lowest density of defect states.

[Third Insulating Film]

The insulating film 216 contains nitrogen or hydrogen. As the insulating film 216, for example, a nitride insulating film can be used. The nitride insulating film can be formed using silicon nitride, silicon nitride oxide, silicon oxynitride, or the like. The hydrogen concentration in the insulating film 216 is preferably higher than or equal to 1×10²² atoms/cm³. Furthermore, the insulating film 216 is in contact with the third region 208 n_2 of the oxide semiconductor film 208. Thus, the concentration of an impurity (nitrogen or hydrogen) in the third region 208 n_2 in contact with the insulating film 216 is increased, leading to an increase in the carrier density of the third region 208 n_2.

[Fourth Insulating Film]

As the insulating film 218, an oxide insulating film can be used. Alternatively, a layered film of an oxide insulating film and a nitride insulating film can be used as the insulating film 218. The insulating film 218 can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn oxide.

Furthermore, the insulating film 218 preferably functions as a barrier film against hydrogen, water, and the like from the outside.

The thickness of the insulating film 218 can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.

<2-3. Structure Example 2 of Transistor>

Next, a structure of a transistor different from that in FIGS. 8A to 8C is described with reference to FIGS. 9A to 9C.

FIG. 9A is a top view of a transistor 200A. FIG. 9B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 9A. FIG. 9C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 9A.

The transistor 200A illustrated in FIGS. 9A to 9C includes a conductive film 206 over the substrate 202; the insulating film 204 over the conductive film 206; the oxide semiconductor film 208 over the insulating film 204; the insulating film 210 over the oxide semiconductor film 208; the conductive film 212 over the insulating film 210; and the insulating film 216 over the insulating film 204, the oxide semiconductor film 208, and the conductive film 212.

The oxide semiconductor film 208 includes the first region 208 n_1 overlapping with the conductive film 212 and being in contact with the insulating film 204, the second region 208 i overlapping with the conductive film 212 and being in contact with the insulating film 210, and the third region 208 n_2 being in contact with the insulating film 216.

The transistor 200A includes the conductive film 206 and an opening 243 in addition to the components of the transistor 200 described above.

Note that the opening 243 is provided in the insulating films 204 and 210. The conductive film 206 is electrically connected to the conductive film 212 through the opening 243. Thus, the same potential is applied to the conductive film 206 and the conductive film 212. Note that different potentials may be applied to the conductive film 206 and the conductive film 212 without providing the opening 243. Alternatively, the conductive film 206 may be used as a light-shielding film without providing the opening 243. When the conductive film 206 is formed using a light-shielding material, for example, light irradiating the second region 208 i from the bottom can be reduced.

In the case of the structure of the transistor 200A, the conductive film 206 functions as a first gate electrode (also referred to as a bottom-gate electrode), the conductive film 212 functions as a second gate electrode (also referred to as a top-gate electrode), the insulating film 204 functions as a first gate insulating film, and the insulating film 210 functions as a second gate insulating film.

The conductive film 206 can be formed using a material similar to the above-described materials of the conductive films 212, 220 a, and 220 b. It is particularly suitable to use a material containing copper as the conductive film 206 because the resistance can be reduced. It is favorable that, for example, each of the conductive films 206, 220 a, and 220 b has a stacked-layer structure in which a copper film is over a titanium nitride film, a tantalum nitride film, or a tungsten film. In that case, by using the transistor 200A as a pixel transistor and/or a driving transistor of a display device, parasitic capacitance generated between the conductive films 206 and 220 a and between the conductive films 206 and 220 b can be reduced. Thus, the conductive films 206, 220 a, and 220 b can be used not only as the first gate electrode, the source electrode, and the drain electrode of the transistor 200A, but also as power source supply wirings, signal supply wirings, connection wirings, or the like of the display device.

In this manner, unlike the transistor 200 described above, the transistor 200A in FIGS. 9A to 9C has a structure in which a conductive film functioning as a gate electrode is provided over and under the oxide semiconductor film 208. As in the transistor 200A, a semiconductor device of one embodiment of the present invention may have a plurality of gate electrodes.

As illustrated in FIGS. 9B and 9C, the oxide semiconductor film 208 faces the conductive film 206 functioning as a first gate electrode and the conductive film 212 functioning as a second gate electrode and is positioned between the two conductive films functioning as the gate electrodes.

Furthermore, the length of the conductive film 212 in the channel width direction is larger than the length of the oxide semiconductor film 208 in the channel width direction. In the channel width direction, the whole oxide semiconductor film 208 is covered with the conductive film 212 with the insulating film 210 placed therebetween. Since the conductive film 212 is connected to the conductive film 206 through the opening 243 provided in the insulating films 204 and 210, a side surface of the oxide semiconductor film 208 in the channel width direction faces the conductive film 212 with the insulating film 210 placed therebetween.

In other words, the conductive film 206 and the conductive film 212 are connected through the opening 243 provided in the insulating films 204 and 210, and each include a region positioned outside an edge portion of the oxide semiconductor film 208.

Such a structure enables the oxide semiconductor film 208 included in the transistor 200A to be electrically surrounded by electric fields of the conductive film 206 functioning as a first gate electrode and the conductive film 212 functioning as a second gate electrode. A device structure of a transistor, like that of the transistor 200A, in which electric fields of a first gate electrode and a second gate electrode electrically surround an oxide semiconductor film in which a channel region is formed can be referred to as a surrounded channel (S-channel) structure.

Since the transistor 200A has the S-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 208 by the conductive film 206 or the conductive film 212; thus, the current drive capability of the transistor 200A can be improved and high on-state current characteristics can be obtained. As a result of the high on-state current, it is possible to reduce the size of the transistor 200A. Furthermore, since the transistor 200A has a structure in which the oxide semiconductor film 208 is surrounded by the conductive film 206 and the conductive film 212, the mechanical strength of the transistor 200A can be increased.

When seen in the channel width direction of the transistor 200A, an opening different from the opening 243 may be formed on the side of the oxide semiconductor film 208 on which the opening 243 is not formed.

When a transistor has a pair of gate electrodes between which a semiconductor film is positioned as in the transistor 200A, one of the gate electrodes may be supplied with a signal A, and the other gate electrode may be supplied with a fixed potential V_(b). Alternatively, one of the gate electrodes may be supplied with the signal A, and the other gate electrode may be supplied with a signal B. Alternatively, one of the gate electrodes may be supplied with a fixed potential V_(a), and the other gate electrode may be supplied with the fixed potential V_(b).

The signal A is, for example, a signal for controlling the on/off state. The signal A may be a digital signal with two kinds of potentials, a potential V and a potential V2 (V1>V2). For example, the potential V1 can be a high power supply potential, and the potential V2 can be a low power supply potential. The signal A may be an analog signal.

The fixed potential V_(b) is, for example, a potential for controlling a threshold voltage V_(thA) of the transistor. The fixed potential V_(b) may be the potential V1 or the potential V2. In that case, a potential generator circuit for generating the fixed potential V_(b) is not necessary, which is preferable. The fixed potential V_(b) may be different from the potential V1 or the potential V2. When the fixed potential V_(b) is low, the threshold voltage V_(thA) can be high in some cases. As a result, the drain current flowing when the gate-source voltage V_(gs) is 0 V can be reduced, and leakage current in a circuit including the transistor can be reduced in some cases. The fixed potential V_(b) may be, for example, lower than the low power supply potential. Meanwhile, a high fixed potential V_(b) can lower the threshold voltage V_(thA) in some cases. As a result, the drain current flowing when the gate-source voltage V_(gs) is a high power supply potential and the operating speed of the circuit including the transistor can be increased in some cases. The fixed potential V_(b) may be, for example, higher than the low power supply potential.

The signal B is, for example, a signal for controlling the on/off state. The signal B may be a digital signal with two kinds of potentials, a potential V3 and a potential V4 (V3>V4). For example, the potential V3 can be a high power supply potential, and the potential V4 can be a low power supply potential. The signal B may be an analog signal.

When both the signal A and the signal B are digital signals, the signal B may have the same digital value as the signal A. In this case, it may be possible to increase the on-state current of the transistor and the operating speed of the circuit including the transistor. Here, the potential V1 and the potential V2 of the signal A may be different from the potential V3 and the potential V4 of the signal B. For example, if a gate insulating film for the gate to which the signal B is input is thicker than a gate insulating film for the gate to which the signal A is input, the potential amplitude of the signal B (V3−V4) may be larger than the potential amplitude of the signal A (V1−V2). In this manner, the influence of the signal A and that of the signal B on the on/off state of the transistor can be substantially the same in some cases.

When both the signal A and the signal B are digital signals, the signal B may have a digital value different from that of the signal A. In this case, the signal A and the signal B can separately control the transistor, and thus, higher performance can be achieved. The transistor which is, for example, an n-channel transistor can function by itself as a NAND circuit, a NOR circuit, or the like in the following case: the transistor is turned on only when the signal A has the potential V1 and the signal B has the potential V3, or the transistor is turned off only when the signal A has the potential V2 and the signal B has the potential V4. The signal B may be a signal for controlling the threshold voltage V_(thA). For example, the potential of the signal B in a period in which the circuit including the transistor operates may be different from the potential of the signal B in a period in which the circuit does not operate. The potential of the signal B may vary depending on the operation mode of the circuit. In this case, the potential of the signal B is not changed as frequently as the potential of the signal A in some cases.

When both the signal A and the signal B are analog signals, the signal B may be an analog signal having the same potential as the signal A, an analog signal whose potential is a constant times the potential of the signal A, an analog signal whose potential is higher or lower than the potential of the signal A by a constant, or the like. In this case, it may be possible to increase the on-state current of the transistor and the operating speed of the circuit including the transistor. The signal B may be an analog signal different from the signal A. In this case, the signal A and the signal B can separately control the transistor, and thus, higher performance can be achieved.

The signal A may be a digital signal, and the signal B may be an analog signal. Alternatively, the signal A may be an analog signal, and the signal B may be a digital signal.

When both of the gate electrodes of the transistor are supplied with the fixed potentials, the transistor can function as an element equivalent to a resistor in some cases. For example, in the case where the transistor is an n-channel transistor, the effective resistance of the transistor can be sometimes low (high) when the fixed potential V_(a) or the fixed potential V_(b) is high (low). When both the fixed potential V_(a) and the fixed potential V_(b) are high (low), the effective resistance can be lower (higher) than that of a transistor with only one gate in some cases.

The other components of the transistor 200A are similar to those of the transistor 200 described above and have similar effects.

An insulating film may further be formed over the transistor 200A. An example of such a case is illustrated in FIGS. 10A and 10B. FIGS. 10A and 10B are cross-sectional views of the transistor 200B. The plan view of the transistor 200B is not illustrated because it is similar to that of the transistor 200A in FIG. 9A.

The transistor 200B illustrated in FIGS. 10A and 10B includes an insulating film 222 over the conductive films 220 a and 220 b and the insulating film 218. The other components of the transistor 200B are similar to those of the transistor 200A and have similar effects.

The insulating film 222 has a function of covering unevenness and the like caused by the transistor or the like. The insulating film 222 has an insulating property and is formed using an inorganic material or an organic material. Examples of the inorganic material include a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, and an aluminum nitride film. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.

<2-4. Structure Example 3 of Transistor>

Next, a structure of a transistor different from that of the transistor 200A in FIGS. 9A to 9C is described with reference to FIGS. 11A and 11B.

FIGS. 11A and 11B are cross-sectional views of a transistor 200C. The plan view of the transistor 200C is not illustrated because it is similar to that of the transistor 200A in FIG. 9A.

The transistor 200C illustrated in FIGS. 11A and 11B is different from the transistor 200A in the layered structure of the conductive film 212, the shape of the conductive film 212, and the shape of the insulating film 210.

The conductive film 212 in the transistor 200C includes a conductive film 212_1 over the insulating film 210 and the conductive film 212_2 over the conductive film 212_1. For example, an oxide conductive film is used as the conductive film 212_1, so that excess oxygen can be added to the insulating film 210. The oxide conductive film can be formed by a sputtering method in an atmosphere containing an oxygen gas. As the oxide conductive film, an oxide including indium and tin, an oxide including tungsten and indium, an oxide including tungsten, indium, and zinc, an oxide including titanium and indium, an oxide including titanium, indium, and tin, an oxide including indium and zinc, an oxide including silicon, indium, and tin, an oxide including indium, gallium, and zinc, or the like can be used, for example.

As illustrated in FIG. 1B, the conductive film 212_2 is connected to the conductive film 206 through the opening 243. By forming the opening 243 after a conductive film to be the conductive film 212_1 is formed, the shape illustrated in FIG. 11B can be obtained. In the case where an oxide conductive film is used as the conductive film 212_1, the structure in which the conductive film 212_2 is connected to the conductive film 206 can decrease the contact resistance between the conductive film 212 and the conductive film 206.

The conductive film 212 and the insulating film 210 in the transistor 200C have a tapered shape. More specifically, the lower edge portion of the conductive film 212 is positioned outside the upper edge portion of the conductive film 212. The lower edge portion of the insulating film 210 is positioned outside the upper edge portion of the insulating film 210. In addition, the lower edge portion of the conductive film 212 is formed in substantially the same position as that of the upper edge portion of the insulating film 210.

As compared with the transistor 200A in which the conductive film 212 and the insulating film 210 have a rectangular shape, the transistor 200C in which the conductive film 212 and the insulating film 210 have a tapered shape is favorable because of better coverage with the insulating film 216.

The other components of the transistor 200C are similar to those of the transistor 200A described above and have similar effects.

<2-5. Manufacturing Method of Semiconductor Device>

Next, an example of a method for manufacturing the transistor 200A illustrated in FIGS. 9A to 9C is described with reference to FIGS. 12A to 12D, FIGS. 13A to 13C, and FIGS. 14A to 14C. Note that FIGS. 12A to 12D, FIGS. 13A to 13C, and FIGS. 14A to 14C are cross-sectional views in the channel length (L) direction and the channel width (W) direction, illustrating the method for manufacturing the transistor 200A.

First, the conductive film 206 is formed over the substrate 202. Then, the insulating film 204 is formed over the substrate 202 and the conductive film 206, and a first oxide semiconductor film and a second oxide semiconductor film are formed over the insulating film 204. Then, the first oxide semiconductor film is processed into an island shape, so that an oxide semiconductor film 208 n_0 is formed. Furthermore, the second oxide semiconductor film is processed into an island shape, so that an oxide semiconductor film 208 i_0 is formed (see FIG. 12A).

The conductive film 206 can be formed using a material selected from the above-mentioned materials. In this embodiment, as the conductive film 206, a layered film of a 50-nm-thick tungsten film and a 400-nm-thick copper film is formed with a sputtering apparatus.

To process the conductive film to be the conductive film 206, a wet etching method and/or a dry etching method can be used. In this embodiment, in the processing of the conductive film into the conductive film 206, the copper film is etched by a wet etching method and then the tungsten film is etched by a dry etching method.

The insulating film 204 can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like as appropriate. In this embodiment, as the insulating film 204, a 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film are formed with a plasma CVD apparatus.

After the insulating film 204 is formed, oxygen may be added to the insulating film 204. As oxygen added to the insulating film 204, an oxygen radical, an oxygen atom, an oxygen atomic ion, an oxygen molecular ion, or the like may be used. Oxygen can be added by an ion doping method, an ion implantation method, a plasma treatment method, or the like. Alternatively, a film that suppresses oxygen release may be formed over the insulating film 204, and then, oxygen may be added to the insulating film 204 through the film.

The film that suppresses oxygen release can be formed using a conductive film or a semiconductor film containing one or more of indium, zinc, gallium, tin, aluminum, chromium, tantalum, titanium, molybdenum, nickel, iron, cobalt, and tungsten.

In the case where oxygen is added by plasma treatment in which oxygen is excited by a microwave to generate high-density oxygen plasma, the amount of oxygen added to the insulating film 204 can be increased.

Furthermore, either or both of the substrate temperature and the percentage of oxygen flow rate in forming the oxide semiconductor film 208 n_0 are preferably lower than those in forming the oxide semiconductor film 208 i_0.

Specifically, the conditions for forming the oxide semiconductor film 208 n_0 are set as follows: the substrate temperature is higher than or equal to room temperature and lower than 150° C., preferably higher than or equal to 100° C. and lower than or equal to 140° C., and the percentage of oxygen flow rate is higher than 0% and lower than 30%. Furthermore, the conditions for forming the oxide semiconductor film 208 i_0 are set as follows: the substrate temperature is higher than or equal to 150° C. and lower than or equal to 300° C., preferably higher than or equal to 160° C. and lower than or equal to 200° C., and the percentage of oxygen flow rate is higher than or equal to 30% and lower than or equal to 100%.

Under the above-described conditions, the oxide semiconductor films having different carrier densities can be stacked. Note that it is more favorable to successively form the oxide semiconductor film 208 n_0 and the oxide semiconductor film 208 i_0 in vacuum because impurities can be prevented from being caught at the interface.

When the oxide semiconductor film 208 is formed while being heated, the crystallinity of the oxide semiconductor film 208 can be increased. However, in the case where a large-sized glass substrate (e.g., the 6th generation to the 10th generation) is used as the substrate 202 and the oxide semiconductor film 208 is formed at a substrate temperature higher than or equal to 200° C. and lower than or equal to 300° C., the substrate 202 might be changed in shape (distorted or warped). In the case where a large-sized glass substrate is used, the change in the shape of the glass substrate can be suppressed by forming the oxide semiconductor film 208 at a substrate temperature higher than or equal to 100° C. and lower than 200° C.

In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used as a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be minimized.

In the case where the oxide semiconductor film is deposited by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the oxide semiconductor film, as much as possible. In particular, the partial pressure of gas molecules corresponding to H₂O (gas molecules corresponding to m/z=18) in the chamber in the standby mode of the sputtering apparatus is preferably lower than or equal to 1×10⁻⁴ Pa, further preferably 5×10⁻⁵ Pa.

In addition, the oxide semiconductor film 208 n_0 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio). The substrate temperature during the formation of the oxide semiconductor film 208 n_0 is 130° C., and oxygen gas at a flow rate of 20 sccm and argon gas at a flow rate of 180 sccm are used as a deposition gas (percentage of oxygen flow rate: 10%).

The oxide semiconductor film 208 i_0 is formed by a sputtering method using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio). The substrate temperature during the formation of the oxide semiconductor film 208 i_0 is 170° C., and oxygen gas at a flow rate of 60 sccm and argon gas at a flow rate of 140 sccm are used as a deposition gas (percentage of oxygen flow rate: 30%).

Note that although the stacked structure of the oxide semiconductor films having different carrier densities was formed by changing the substrate temperature and the percentage of oxygen flow rate between the oxide semiconductor films 208 n_0 and 208 i_0 in the above-described example, the method for forming the structure is not limited to this example. For example, an impurity element may be added in formation of the oxide semiconductor film 208 n_0 to make the carrier density of the oxide semiconductor film 208 n_0 different from that of the oxide semiconductor film 208 i_0. Examples of the impurity element include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, and a rare gas element.

Among the above-described elements, nitrogen is particularly preferable as the impurity element added to the oxide semiconductor film 208 n_0. For example, nitrogen can be added to the oxide semiconductor film 208 n_0 by using argon gas and nitrogen gas as a deposition gas or using argon gas and dinitrogen monoxide as a deposition gas in forming the oxide semiconductor film 208 n_0.

In the case where an impurity element is used to form the oxide semiconductor film 208 n_0, it is favorable to independently provide a chamber for forming the oxide semiconductor film 208 n_0 in order to prevent the impurity element from entering a film into which the impurity element is preferably not added, e.g., the oxide semiconductor film 208 i_0.

After the oxide semiconductor film 208 n_0 is formed, an impurity element may be added to the oxide semiconductor film 208 n_0. As a method for adding an impurity element after formation of the oxide semiconductor film 208 n_0, doping treatment or plasma treatment can be used, for example.

After the oxide semiconductor film 208 n_0 and the oxide semiconductor film 208 i_0 are formed, the oxide semiconductor film 208 n_0 and the oxide semiconductor film 208 i_0 may be dehydrated or dehydrogenated by heat treatment. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, higher than or equal to 250° C. and lower than or equal to 450° C., or higher than or equal to 300° C. and lower than or equal to 450° C.

The heat treatment can be performed in an inert gas atmosphere containing nitrogen or a rare gas such as helium, neon, argon, xenon, or krypton. Alternatively, the heat treatment may be performed in an inert gas atmosphere first, and then, in an oxygen atmosphere. It is preferable that the above inert gas atmosphere and the above oxygen atmosphere do not contain hydrogen, water, and the like. The treatment time may be longer than or equal to 3 minutes and shorter than or equal to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for the heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened.

By depositing the oxide semiconductor film while it is heated or by performing heat treatment after the formation of the oxide semiconductor film, the hydrogen concentration in the oxide semiconductor film, which is measured by SIMS, can be 5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³ or lower, 5×10¹⁸ atoms/cm³ or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷ atoms/cm³ or lower, or 1×10¹⁶ atoms/cm³ or lower.

Next, an insulating film 210_0 is formed over the insulating film 204 and the oxide semiconductor film 208 i_0 (see FIG. 12B).

As the insulating film 210_0, a silicon oxide film or a silicon oxynitride film can be formed with a plasma-enhanced chemical vapor deposition apparatus (a PECVD apparatus or simply referred to as a plasma CVD apparatus). In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As examples of the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given.

A silicon oxynitride film having few defects can be formed as the insulating film 110_0 with a plasma CVD apparatus under the conditions that the flow rate of the oxidizing gas is more than 20 times and less than 100 times, or more than or equal to 40 times and less than or equal to 80 times the flow rate of the deposition gas and that the pressure in a treatment chamber is lower than 100 Pa, or lower than or equal to 50 Pa.

As the insulating film 210_0, a dense silicon oxide film or a dense silicon oxynitride film can be formed under the following conditions: the substrate placed in a vacuum-evacuated treatment chamber of a plasma CVD apparatus is held at a temperature higher than or equal to 280° C. and lower than or equal to 400° C., the pressure in the treatment chamber into which a source gas is introduced is set to be higher than or equal to 20 Pa and lower than or equal to 250 Pa, preferably higher than or equal to 100 Pa and lower than or equal to 250 Pa, and a high-frequency power is supplied to an electrode provided in the treatment chamber.

The insulating film 210_0 may be formed by a plasma CVD method using a microwave. A microwave refers to a wave in the frequency range of 300 MHz to 300 GHz. In a microwave, electron temperature and electron energy are low. Furthermore, in supplied power, the proportion of power used for acceleration of electrons is low, and therefore, much more power can be used for dissociation and ionization of molecules. Thus, plasma with a high density (high-density plasma) can be excited. This method causes little plasma damage to the deposition surface or a deposit, so that the insulating film 210_0 having few defects can be formed.

Alternatively, the insulating film 210_0 can also be formed by a CVD method using an organosilane gas. As the organosilane gas, the following silicon-containing compound can be used: tetraethyl orthosilicate (TEOS) (chemical formula: Si(OC₂H₅)₄), tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H)₃), trisdimethylaminosilane (SiH(N(CH₃)₂)₃), or the like. By a CVD method using an organosilane gas, the insulating film 210_0 having high coverage can be formed.

In this embodiment, as the insulating film 210_0, a 100-nm-thick silicon oxynitride film is formed with a plasma CVD apparatus.

Subsequently, a mask is formed by lithography in a desired position over the insulating film 210_0, and then, the insulating film 210_0 and the insulating film 204 are partly etched, so that the opening 243 reaching the conductive film 206 is formed (see FIG. 12C).

To form the opening 243, a wet etching method and/or a dry etching method can be used. In this embodiment, the opening 243 is formed by a dry etching method.

Next, a conductive film 212_0 is formed over the conductive film 206 and the insulating film 210_0 so as to cover the opening 243. In the case where a metal oxide film is used as the conductive film 212_0, for example, oxygen might be added from the conductive film 212_0 to the insulating film 210_0 during the formation of the conductive film 212_0 (see FIG. 12D).

In FIG. 12D, oxygen added to the insulating film 210_0 is schematically shown by arrows. Furthermore, the conductive film 212_0 formed to cover the opening 243 is electrically connected to the conductive film 206.

In the case where a metal oxide film is used as the conductive film 212_0, the conductive film 212_0 is preferably formed by a sputtering method in an atmosphere containing an oxygen gas. Formation of the conductive film 212_0 in an atmosphere containing an oxygen gas allows suitable addition of oxygen to the insulating film 210_0. Note that a method for forming the conductive film 212_0 is not limited to a sputtering method, and other methods such as an ALD method may be used.

In this embodiment, a 100-nm-thick IGZO film containing an In—Ga—Zn oxide (In:Ga:Zn=4:2:4.1 [atomic ratio]) is formed as the conductive film 212_0 by a sputtering method. Note that oxygen addition treatment may be performed on the insulating film 210_0 before or after the formation of the conductive film 212_0. The oxygen addition treatment can be performed similarly to the oxygen addition that can be performed after the formation of the insulating film 204.

Subsequently, the mask 240 is formed by a lithography process in a desired position over the conductive film 212_0 (see FIG. 13A).

Next, etching is performed from above the mask 240 to process the conductive film 212_0 and the insulating film 210_0. After the processing of the conductive film 212_0 and the insulating film 210_0, the mask 240 is removed. As a result of the processing of the conductive film 212_0 and the insulating film 210_0, the island-shaped conductive film 212 and the island-shaped insulating film 210 are formed (see FIG. 13B).

In this embodiment, the conductive film 212_0 and the insulating film 210_0 are processed by a dry etching method.

In the processing of the conductive film 212_0 and the insulating film 210_0, the thickness of the oxide semiconductor film 208 i_0 in a region not overlapping with the conductive film 212 is decreased in some cases. In other cases, in the processing of the conductive film 212_0 and the insulating film 210_0, the thickness of the insulating film 204 in a region not overlapping with the oxide semiconductor film 208 i_0 is decreased. In the processing of the conductive film 212_0 and the insulating film 210_0, an etchant or an etching gas (e.g., chlorine) might be added to the oxide semiconductor film 208 i_0 or the constituent element of the conductive film 212_0 or the insulating film 210_0 might be added to the oxide semiconductor film 208 i_0.

Next, the insulating film 216 is formed over the insulating film 204, the oxide semiconductor film 208 i_0, and the conductive film 212, whereby the oxide semiconductor film 208 i_0, which is in contact with the insulating film 216, and the oxide semiconductor film 208 n_0 become the third region 208 n_2. Furthermore, the oxide semiconductor film 208 i_0 in contact with the insulating film 210 becomes the second region 208 i. In addition, the oxide semiconductor film 208 n_0 that is in contact with the insulating film 204 and overlaps with the conductive film 212 becomes the first region 208 n_1. Thus, the oxide semiconductor film 208 including the first region 208 n_1, the second region 208 i, and the third region 208 n_2 is formed (see FIG. 13C).

The insulating film 216 can be formed using a material selected from the above-mentioned materials. In this embodiment, as the insulating film 216, a 100-nm-thick silicon nitride oxide film is formed with a PECVD apparatus. In the formation of the silicon nitride oxide film, plasma treatment and deposition treatment are performed at 220° C. The plasma treatment is performed before deposition under the following conditions: an argon gas at a flow rate of 100 sccm is introduced into a chamber, the pressure in the chamber is set to 40 Pa, and power of 1000 W is supplied to an RF power source (27.12 MHz). The deposition treatment is performed under the following conditions: a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm are introduced into the chamber; the pressure in the chamber is set to 100 Pa; and power of 1000 W is supplied to the RF power source (27.12 MHz).

When a silicon nitride oxide film is used as the insulating film 216, nitrogen or hydrogen in the silicon nitride oxide film can be supplied to the third region 208 n_2 in contact with the insulating film 216. Moreover, when the temperature in forming the insulating film 216 is the above-mentioned temperature, release of excess oxygen contained in the insulating film 210 to the outside can be suppressed.

Next, the insulating film 218 is formed over the insulating film 216 (see FIG. 14A).

The insulating film 218 can be formed using a material selected from the above-mentioned materials. In this embodiment, as the insulating film 218, a 300-nm-thick silicon oxynitride film is formed with a plasma CVD apparatus.

Then, a mask is formed over desired positions of the insulating film 218 by lithography, and the insulating film 218 and the insulating film 216 are partly etched. Thus, the openings 241 a and 241 b reaching the third region 208 n_2 are formed (see FIG. 14B).

To etch the insulating films 218 and 216, a wet etching method and/or a dry etching method can be used. In this embodiment, the insulating films 218 and 216 are processed by a dry etching method.

Next, a conductive film is formed over the third region 208 n_2 and the insulating film 218 to cover the openings 241 a and 241 b, and processed into desired shapes, so that the conductive films 220 a and 220 b are formed (see FIG. 14C).

The conductive films 220 a and 220 b can be formed using a material selected from the above-mentioned materials. In this embodiment, as the conductive films 220 a and 220 b, a layered film including a 50-nm-thick tungsten film and a 400-nm-thick copper film is formed with a sputtering apparatus.

To process the conductive film to be the conductive films 220 a and 220 b, a wet etching method and/or a dry etching method can be used. In this embodiment, in the processing of the conductive film into the conductive films 220 a and 220 b, the copper film is etched by a wet etching method and then the tungsten film is etched by a dry etching method.

Through the above process, the transistor 200A in FIGS. 9A to 9C can be fabricated.

Note that the films included in the transistor 200A (the insulating film, the metal oxide film, the oxide semiconductor film, the conductive film, and the like) can be formed by, other than the above methods, a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, or an ALD method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a plasma-enhanced chemical vapor deposition (PECVD) method are typical examples of the film formation method, a thermal CVD method may be used. As an example of a thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method can be given.

Deposition by a thermal CVD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas and an oxidizer are supplied to the chamber at a time and react with each other in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, a thermal CVD method has an advantage that no defect due to plasma damage is caused.

The films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films that are described above can be formed by a thermal CVD method such as an MOCVD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium (In(CH₃)₃), trimethylgallium (Ga(CH₃)₃), and dimethylzinc (Zn(CH₃)₂) are used. Without limitation to the above combination, triethylgallium (Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc (Zn(C₂H₅)₂) can be used instead of dimethylzinc.

In the case where a hafnium oxide film is formed with a deposition apparatus employing an ALD method, two kinds of gases are used, namely, ozone (O) as an oxidizer and a source gas that is obtained by vaporizing liquid containing a solvent and a hafnium precursor (hafnium alkoxide or hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH, Hf[N(CH₃)₂]₄) or tetrakis(ethylmethylamide)hafnium).

In the case where an aluminum oxide film is formed with a deposition apparatus employing an ALD method, two kinds of gases are used, namely, H₂O as an oxidizer and a source gas that is obtained by vaporizing liquid containing a solvent and an aluminum precursor (e.g., trimethylaluminum (TMA, Al(CH₃)₃)). Examples of another material include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

In the case where a silicon oxide film is formed with a deposition apparatus employing an ALD method, hexachlorodisilane is adsorbed on a surface on which a film is to be formed, and radicals of an oxidizing gas (O₂ or dinitrogen monoxide) are supplied to react with the adsorbate.

In the case where a tungsten film is formed with a deposition apparatus employing an ALD method, a WF₆ gas and a B₂H₆ gas are sequentially introduced to form an initial tungsten film, and then, a WF₆ gas and an H₂ gas are used to form a tungsten film. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

In the case where an oxide semiconductor film such as an In—Ga—Zn—O film is formed with a deposition apparatus employing an ALD method, an In(CH₃)₃ gas and an O₃ gas are used to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are used to form a Ga—O layer, and then, a Zn(CH₃)₂ gas and an O₃ gas are used to form a Zn—O layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by using these gases. Note that although an H₂O gas that is obtained by bubbling water with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an 03 gas, which does not contain H.

One embodiment of the present invention is not limited to the example described in this embodiment, in which the transistor includes an oxide semiconductor film. In one embodiment of the present invention, the transistor does not necessarily include an oxide semiconductor film. For example, a channel region, the vicinity of the channel region, a source region, or a drain region of the transistor may be formed using a material containing silicon (Si), germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs), or the like.

Note that the structure and method described in this embodiment can be used in appropriate combination with the structure and method described in any of the other embodiments.

Embodiment 3

In this embodiment, the composition and structure of an oxide semiconductor that can be used in one embodiment of the present invention are described with reference to FIGS. 15A to 15C, FIG. 16, FIGS. 17A to 17E, FIGS. 18A to 18E, FIGS. 19A to 19D, FIGS. 20A and 20B, and FIG. 21.

<3-1. Composition of Oxide Semiconductor>

The composition of an oxide semiconductor is described below.

An oxide semiconductor preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the like may be contained.

Here, the case where an oxide semiconductor contains indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like.

Alternatively, the element M can be boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the like. Note that two or more of the above elements may be used in combination as the element M.

First, preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor according to an embodiment of the present invention are described with reference to FIGS. 15A to 15C. Note that the proportion of oxygen atoms is not shown. The terms of the atomic ratio of indium, the element M, and zinc contained in the oxide semiconductor are denoted by [In], [M], and [Zn], respectively.

In FIGS. 15A to 15C, broken lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):1, where −1≦α≦1, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):5.

Dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is 1:1:β, where β≧0, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:4:β, a line where the atomic ratio [In]:[M]:[Zn] is 2:1:β, and a line where the atomic ratio [In]:[M]:[Zn] is 5:1:β.

Dashed-double dotted lines is a line indicating the atomic ratio [In]:[M]:[Zn]=(1+γ):2:(1−γ) (−1≦γ≦1). The oxide semiconductor shown in FIGS. 15A to 15C with an atomic ratio of [In]:[M]:[Zn]=0:2:1 or an atomic ratio that is in the neighborhood thereof is likely to have a spinel crystal structure.

FIGS. 15A and 15B show examples of the preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor in one embodiment of the present invention.

FIG. 16 shows an example of the crystal structure of InMZnO₄ whose atomic ratio [In]:[M]:[Zn] is 1:1:1. The crystal structure shown in FIG. 16 is InMZnO₄ observed from a direction parallel to a b-axis. Note that a metal element in a layer that contains M, Zn, and oxygen (hereinafter, this layer is referred to as an “(M,Zn) layer”) in FIG. 16 represents the element M or zinc. In that case, the proportion of the element M is the same as the proportion of zinc. The element M and zinc can be replaced with each other, and their arrangement is random.

InMZnO₄ has a layered crystal structure (also referred to as a layered structure) and includes one layer that contains indium and oxygen (hereinafter referred to as an In layer) for every two (M,Zn) layers that contain the element M, zinc, and oxygen, as shown in FIG. 16.

Indium and the element M can be replaced with each other. Thus, when the element M in the (M,Zn) layer is replaced with indium, the layer can also be referred to as an (In,M,Zn) layer. In that case, a layered structure that contains one In layer for every two (In,M,Zn) layers is obtained.

An oxide whose atomic ratio [In]:[M]:[Zn] is 1:1:2 has a layered structure that contains one In layer for every three (M,Zn) layers. In other words, if [Zn] is higher than [In] and [M], the proportion of the (M,Zn) layer to the In layer becomes higher when the oxide is crystallized.

Note that in the case where the number of (M,Zn) layers with respect to one In layer is not an integer in the oxide, the oxide might have a plurality of kinds of layered structures where the number of (M,Zn) layers with respect to one In layer is an integer. For example, in the case of [In]:[M]:[Zn]=1:1:1.5, the oxide may have a mix of a layered structure including one In layer for every two (M,Zn) layers and a layered structure including one In layer for every three (M,Zn) layers.

For example, in the case where the oxide is formed with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of a target is formed. In particular, [Zn] in the film might be smaller than [Zn] in the target depending on the substrate temperature in deposition.

A plurality of phases (e.g., two phases or three phases) exist in the oxide in some cases. For example, with an atomic ratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinel crystal structure and a layered crystal structure are likely to exist. In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of a bixbyite crystal structure and a layered crystal structure are likely to exist. In the case where a plurality of phases exist in the oxide, a grain boundary might be formed between different crystal structures.

In contrast, when the indium content and the zinc content in an oxide become lower, carrier mobility becomes lower. Thus, with atomic ratios of [In]:[M]:[Zn]=0:1:0 and in the vicinity thereof (e.g., a region C in FIG. 15C), insulation performance becomes better.

Accordingly, an oxide in one embodiment of the present invention preferably has an atomic ratio represented by a region A in FIG. 15A. With the atomic ratio, a layered structure with high carrier mobility and a few grain boundaries is easily obtained.

A region B in FIG. 15B represents an atomic ratio of [In]:[M]:[Zn]=4:2:3 or 4:2:4.1 and the vicinity thereof. The vicinity includes an atomic ratio of [In]:[M]:[Zn]=5:3:4. An oxide with an atomic ratio represented by the region B is an excellent oxide that has particularly high crystallinity and high carrier mobility.

Note that a condition where an oxide forms a layered structure is not uniquely determined by an atomic ratio. There is a difference in the degree of difficulty in forming a layered structure among atomic ratios. Even with the same atomic ratio, whether a layered structure is formed or not depends on a formation condition. Thus, the illustrated regions each represent an atomic ratio with which an oxide has a layered structure, and boundaries of the regions A to C are not clear.

<3-2. Structure in which Oxide Semiconductor is Used in Transistor>

Next, a structure in which the oxide semiconductor is used in a transistor is described.

Note that when the oxide semiconductor is used for a transistor, carrier scattering or the like at a grain boundary can be reduced; thus, the transistor can have high field-effect mobility. In addition, the transistor can have high reliability.

An oxide semiconductor with low carrier density is preferably used for a channel region of the transistor. For example, an oxide semiconductor whose carrier density is lower than 8×10¹⁸/cm³, preferably lower than 1×10¹¹/cm³, further preferably lower than 1×10¹⁰/cm³, and greater than or equal to 1×10⁻⁹/cm³ is used.

A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources and thus can have a low carrier density. The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has a low density of defect states and accordingly has a low density of trap states in some cases.

Charge trapped by the trap states in the oxide semiconductor takes a long time to be released and may behave like fixed charge. Thus, a transistor whose channel region is formed in an oxide semiconductor having a high density of trap states has unstable electrical characteristics in some cases.

In order to obtain stable electrical characteristics of the transistor, it is effective to reduce the concentration of impurities in the oxide semiconductor. In addition, in order to reduce the concentration of impurities in the oxide semiconductor, the concentration of impurities in a film that is adjacent to the oxide semiconductor is preferably reduced. Examples of impurities include hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, and silicon.

Here, the influence of impurities in the oxide semiconductor is described.

When silicon or carbon that is one of Group 14 elements is contained in the oxide semiconductor, defect states are formed. Thus, the concentration of silicon or carbon in the oxide semiconductor and around an interface with the oxide semiconductor (measured by secondary ion mass spectrometry (SIMS)) is set lower than or equal to 2×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor contains alkali metal or alkaline earth metal, defect states are formed and carriers are generated, in some cases. Thus, a transistor including an oxide semiconductor that contains alkali metal or alkaline earth metal is likely to be normally-on. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor. Specifically, the concentration of alkali metal or alkaline earth metal in the oxide semiconductor measured by SIMS is set lower than or equal to 1×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When the oxide semiconductor contains nitrogen, the oxide semiconductor easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor whose semiconductor includes an oxide semiconductor that contains nitrogen is likely to be normally-on. For this reason, nitrogen in the oxide semiconductor is preferably reduced as much as possible; for example, the concentration of nitrogen in the oxide semiconductor, which is measured by SIMS, can be lower than 6×10²⁰ atoms/cm³, preferably lower than 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³, still further preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy, in some cases. Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor that contains hydrogen is likely to be normally-on. Accordingly, it is preferable that hydrogen in the oxide semiconductor be reduced as much as possible. Specifically, the hydrogen concentration of the oxide semiconductor measured by SIMS is set lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, and still further preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor with sufficiently reduced impurity concentration is used for a channel formation region in a transistor, the transistor can have stable electrical characteristics.

The energy gap of the oxide semiconductor film is preferably 2 eV or more, 2.5 eV or more, or 3 eV or more.

The thickness of the oxide semiconductor film is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 60 nm.

When the oxide semiconductor film is an In-M-Zn oxide, as the atomic ratio of metal elements in a sputtering target used for formation of the In-M-Zn oxide, In:M:Zn=1:1:0.5, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5, In:M:Zn=2:1:2.3, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, In:M:Zn=5:1:7, or the like is preferable.

Note that the atomic ratios of metal elements in the formed oxide semiconductor films may each vary from the above atomic ratio of metal elements in the sputtering target within a range of approximately ±40%. For example, when a sputtering target with an atomic ratio of In:Ga:Zn=4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the oxide semiconductor film may be approximately 4:2:3. In the case where a sputtering target whose atomic ratio of In to Ga and Zn is 5:1:7 is used, the atomic ratio of In to Ga and Zn in the formed oxide semiconductor film may be approximately 5:1:6.

<3-3. Structure of Oxide Semiconductor>

Next, a structure of an oxide semiconductor is described.

An oxide semiconductor is classified into a single-crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of the non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of the crystalline oxide semiconductor include a single-crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.

An amorphous structure is generally thought to be isotropic and have no non-uniform structure, to be metastable and have no fixed atomic arrangement, to have a flexible bond angle, and to have a short-range order but have no long-range order, for example.

In other words, a stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. In contrast, an a-like OS, which is not isotropic, has an unstable structure that includes a void. Because of its instability, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.

[CAAC-OS]

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors and has a plurality of c-axis aligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal, which is classified into the space group R-3m, is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 17A. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment and that the c-axes are aligned in the direction substantially perpendicular to a surface over which the CAAC-OS is formed (also referred to as a formation surface) or a top surface of the CAAC-OS. Note that a peak sometimes appears at 2θ of around 36° in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° is attributed to a crystal structure classified into the space group Fd-3m; thus, this peak is preferably not exhibited in the CAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on the CAAC-OS in the direction parallel to the formation surface, a peak appears at 2θ of around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. When analysis (0 scan) is performed with 2θ fixed at around 56° while the sample is rotated around a normal vector to the sample surface as an axis (0 axis), as shown in FIG. 17B, a peak is not clearly observed. In contrast, in the case where single-crystal InGaZnO₄ is subjected to φ scan with 2θ fixed at around 56°, as shown in FIG. 17C, six peaks that are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of the a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in the direction parallel to the formation surface of the CAAC-OS, a diffraction pattern (also referred to as a selected-area electron diffraction pattern) in FIG. 17D can be obtained. This diffraction pattern includes spots derived from the (009) plane of the InGaZnO₄ crystal. Thus, the results of electron diffraction also indicate that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in the direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 17E shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in the direction perpendicular to the sample surface. In FIG. 17E, a ring-like diffraction pattern is observed. Thus, the results of electron diffraction using an electron beam with a probe diameter of 300 nm also indicate that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in FIG. 17E is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. The second ring in FIG. 17E is considered to be derived from the (110) plane and the like.

In a combined analysis image (also referred to as a high-resolution transmission electron microscope (TEM) image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a TEM, a plurality of pellets can be observed. However, even in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed in some cases. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

FIG. 18A shows a high-resolution TEM image of a cross section of the CAAC-OS that is observed in the direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be observed with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 18A shows pellets in which metal atoms are arranged in a layered manner. FIG. 18A proves that the size of a pellet is greater than or equal to 1 nm or greater than or equal to 3 nm. Thus, the pellet can also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS can also be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC). A pellet reflects unevenness of a formation surface or a top surface of the CAAC-OS and is parallel to the formation surface or the top surface of the CAAC-OS.

FIGS. 18B and 18C show Cs-corrected high-resolution TEM images of a plane of the CAAC-OS observed in the direction substantially perpendicular to the sample surface. FIGS. 18D and 18E are images obtained by image processing of FIGS. 18B and 18C. The method of image processing is as follows. The image in FIG. 18B is subjected to fast Fourier transform (FFT) to obtain an FFT image. Then, mask processing is performed on the obtained FFT image such that part in the range of 2.8 nm⁻¹ to 5.0 nm⁻¹ from the reference point is left. After the mask processing, the FFT image is subjected to inverse fast Fourier transform (IFFT) to obtain a processed image. The image obtained in this manner is referred to as an FFT filtering image. The FFT filtering image is a Cs-corrected high-resolution TEM image from which a periodic component is extracted and shows a lattice arrangement.

In FIG. 18D, a portion in which the lattice arrangement is broken is shown by dashed lines. A region surrounded by dashed lines corresponds to one pellet. The portion denoted by the dashed lines is a junction of pellets. The dashed lines draw a hexagon, which means that the pellet has a hexagonal shape. Note that the shape of the pellet is not always a regular hexagon but is a non-regular hexagon in many cases.

In FIG. 18E, a dotted line denotes a portion where the direction of a lattice arrangement changes between a region with a well lattice arrangement and another region with a well lattice arrangement, and a dashed line denotes the change in the direction of the lattice arrangement. A clear crystal grain boundary cannot be observed even in the vicinity of the dotted line. When a lattice point in the vicinity of the dotted line is regarded as a center and surrounding lattice points are joined, a distorted hexagon, a distorted pentagon, and/or a distorted heptagon can be formed, for example. That is, a lattice arrangement is distorted so that formation of a crystal grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of the atomic arrangement in an a-b plane direction, the interatomic bond distance changed by substitution of a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets (nanocrystals) are connected in the a-b plane direction, and its crystal structure has distortion. For this reason, the CAAC-OS can also be referred to as an oxide semiconductor including a c-axis-aligned a-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has few impurities and defects (e.g., oxygen vacancies).

Note that an impurity means an element other than the main components of an oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (e.g., silicon) having stronger bonding force to oxygen than a metal element constituting a part of an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in a disordered atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. For example, impurities contained in an oxide semiconductor might serve as a carrier trap or a carrier generation source. For example, oxygen vacancies in an oxide semiconductor might serve as a carrier trap or serve as a carrier generation source when hydrogen is captured therein.

The CAAC-OS having a small amount of impurities and oxygen vacancies is an oxide semiconductor with low carrier density. Specifically, an oxide semiconductor with a carrier density of lower than 8×10¹¹ cm⁻³, preferably lower than 1×10¹¹ cm⁻³, further preferably lower than 1×10¹⁰ cm⁻³, and higher than or equal to 1×10⁻⁹ cm⁻³ can be used. Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.

[nc-OS]

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OS is analyzed by an out-of-plane method, a peak indicating orientation does not appear. That is, a crystal of an nc-OS does not have orientation.

For example, when an electron beam with a probe diameter of 50 nm is incident on a 34-nm-thick region of a thinned nc-OS including an InGaZnO₄ crystal in the direction parallel to the formation surface, a ring-like diffraction pattern (nanobeam electron diffraction pattern) shown in FIG. 19A is observed. FIG. 19B shows a diffraction pattern (nanobeam electron diffraction pattern) obtained when an electron beam with a probe diameter of 1 nm is incident on the same sample. In FIG. 19B, a plurality of spots are observed in a ring-like region. Thus, ordering in an nc-OS is not observed with an electron beam with a probe diameter of 50 nm but is observed with an electron beam with a probe diameter of 1 nm.

When an electron beam with a probe diameter of 1 nm is incident on a region with a thickness less than 10 nm, an electron diffraction pattern in which spots are arranged in an approximately regular hexagonal shape as shown in FIG. 19C is observed in some cases. This means that an nc-OS has a well-ordered region, that is, a crystal, in the thickness range of less than 10 nm. Note that an electron diffraction pattern having regularity is not observed in some regions because crystals are aligned in various directions.

FIG. 19D shows a Cs-corrected high-resolution TEM image of a cross section of an nc-OS observed in the direction substantially parallel to the formation surface. In the high-resolution TEM image, the nc-OS has a region in which a crystal part is observed as indicated by additional lines and a region in which a crystal part is not clearly observed. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, specifically greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm may be referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Thus, a crystal part of the nc-OS may be referred to as a pellet in the following description.

As described above, in the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets (nanocrystals), the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has higher regularity than an amorphous oxide semiconductor. Thus, the nc-OS has a lower density of defect states than the a-like OS and the amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

[a-like OS]

An a-like OS has a structure between the structure of an nc-OS and the structure of an amorphous oxide semiconductor.

FIGS. 20A and 20B show high-resolution cross-sectional TEM images of an a-like OS. The high-resolution cross-sectional TEM image of the a-like OS in FIG. 20A is taken at the start of the electron irradiation. The high-resolution cross-sectional TEM image of the a-like OS in FIG. 20B is taken after the irradiation with electrons (e⁻) at 4.3×10⁸ e⁻/nm². FIGS. 20A and 20B show that striped bright regions extending vertically are observed in the a-like OS from the start of the electron irradiation. It can be also found that the shape of the bright region changes after the electron irradiation. Note that the bright region is presumably a void or a low-density region.

The a-like OS has an unstable structure because it includes a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained.

The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion in which the spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ in the following description. Each lattice fringe corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 21 shows a change in the average size of crystal parts (at 22 points to 30 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 21 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose in obtaining TEM images, for example. As shown in FIG. 21, a crystal part with a size of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part sizes in the nc-OS and the CAAC-OS show few changes from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 21, the crystal part sizes in the nc-OS and the CAAC-OS are approximately 1.3 nm and approximately 1.8 nm, respectively, regardless of the cumulative electron dose. For the electron beam irradiation and TEM observation, a Hitachi H-9000NAR transmission electron microscope was used. The conditions of the electron beam irradiation were as follows: the accelerating voltage was 300 kV; the current density was 6.7×10⁵ e⁻/(nm²-s); and the diameter of an irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS may be induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. That is, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it includes a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single-crystal oxide semiconductor having the same composition. The density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 92.3% and lower than 100% of the density of the single-crystal oxide semiconductor having the same composition. It is difficult to deposit an oxide semiconductor having a density lower than 78% of the density of the single-crystal oxide semiconductor.

For example, in the case of an oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of single-crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³, for example. In the case of the oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³, for example.

In the case where an oxide semiconductor having a certain composition does not exist in a single-crystal state, single-crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate a density equivalent to that of a single-crystal oxide semiconductor with the desired composition. The density of a single-crystal oxide semiconductor having the desired composition may be calculated using a weighted average with respect to the combination ratio of the single-crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single-crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked film including two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

The structures described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 4

In this embodiment, examples of a display device that includes the transistor described in the above embodiments are described below with reference to FIG. 22, FIG. 23, FIG. 24, FIG. 25, FIG. 26, FIG. 27, FIGS. 28A to 28C, and FIG. 29.

FIG. 22 is a top view illustrating an example of a display device. A display device 700 in FIG. 22 includes a pixel portion 702 provided over a first substrate 701, a source driver circuit portion 704 and a gate driver circuit portion 706 that are provided over the first substrate 701, a sealant 712 provided to surround the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706, and a second substrate 705 provided to face the first substrate 701. The first substrate 701 and the second substrate 705 are sealed with the sealant 712. That is, the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 are enclosed by the first substrate 701, the sealant 712, and the second substrate 705. Although not illustrated in FIG. 22, a display element is provided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminal portion 708 that is electrically connected to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 is provided in a region different from the region that is over the first substrate 701 and surrounded by the sealant 712. Furthermore, an FPC 716 is connected to the FPC terminal portion 708, and a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706. Furthermore, a signal line 710 is connected to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708. Through the signal line 710, a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708.

A plurality of gate driver circuit portions 706 may be provided in the display device 700. The structure of the display device 700 is not limited to the example shown here, in which the source driver circuit portion 704 and the gate driver circuit portion 706 as well as the pixel portion 702 are formed over the first substrate 701. For example, only the gate driver circuit portion 706 may be formed over the first substrate 701, or only the source driver circuit portion 704 may be formed over the first substrate 701. In this case, a substrate over which a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver circuit board formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be formed on the first substrate 701. Note that there is no particular limitation on the method for connecting the separately prepared driver circuit board, and a chip on glass (COG) method, a wire bonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 included in the display device 700 include a plurality of transistors.

The display device 700 can include a variety of elements. As examples of the elements, electroluminescent (EL) element (e.g., an EL element containing organic and inorganic materials, an organic EL element, an inorganic EL element, or an LED), a light-emitting transistor element (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, an electronic ink display, an electrophoretic element, an electrowetting element, a plasma display panel (PDP), micro electro mechanical systems (MEMS) display (e.g., a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS) element, or an interferometric modulator display (IMOD) element), and a piezoelectric ceramic display can be given.

An example of a display device including an EL element is an EL display. Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). An example of a display device including a liquid crystal element is a liquid crystal display (a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including an electronic ink display or an electrophoretic element is electronic paper. In a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes may function as reflective electrodes. For example, some or all of pixel electrodes may contain aluminum, silver, or the like. In this case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.

As a display system of the display device 700, a progressive system, an interlace system, or the like can be employed. Furthermore, color elements controlled in pixels at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of an R pixel, a G pixel, a B pixel, and a W (white) pixel may be used. Alternatively, a color element may be composed of two colors of R, G, and B as in PenTile layout. The two colors may differ depending on the color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Note that the size of a display region may differ between dots of color elements. One embodiment of the disclosed invention is not limited to a color display device; the disclosed invention can also be applied to a monochrome display device.

A coloring layer (also referred to as a color filter) may be used to obtain a full-color display device in which white light (W) is used for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp). For example, a red (R) coloring layer, a green (G) coloring layer, a blue (B) coloring layer, and a yellow (Y) coloring layer can be combined as appropriate. With the use of the coloring layer, high color reproducibility can be obtained as compared with the case without the coloring layer. Here, by providing a region with a coloring layer and a region without a coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without a coloring layer, a decrease in the luminance of a bright image due to the coloring layer can be suppressed, and approximately 20% to 30% of power consumption can be reduced in some cases. In the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light in their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption may be further reduced as compared with the case of using a coloring layer.

As a coloring system, any of the following systems may be used: the above-described color filter system in which part of white light is converted into red light, green light, and blue light through color filters; a three-color system in which red light, green light, and blue light are used; and a color conversion system or a quantum dot system in which part of blue light is converted into red light or green light.

In this embodiment, a structure including a liquid crystal element as a display element and a structure including an EL element as a display element are described with reference to FIG. 23 to FIG. 25. FIG. 23 and FIG. 24 are each a cross-sectional view taken along dashed-dotted line Q-R in FIG. 22 and illustrate the structure including a liquid crystal element as a display element. FIG. 25 is a cross-sectional view taken along dashed-dotted line Q-R in FIG. 22 and illustrates the structure including an EL element as a display element.

Portions common to FIG. 23 to FIG. 25 are described first, and then, different portions are described.

<4-1. Portions Common to Display Devices>

The display device 700 in FIG. 23 to FIG. 25 includes a lead wiring portion 711, the pixel portion 702, the source driver circuit portion 704, and the FPC terminal portion 708. The lead wiring portion 711 includes the signal line 710. The pixel portion 702 includes a transistor 750 and a capacitor 790. The source driver circuit portion 704 includes a transistor 752.

The transistor 750 and the transistor 752 each have a structure similar to that of the transistor 100 described above. Note that the transistor 750 and the transistor 752 may each have the structure of any of the other transistors described in the above embodiments.

The transistor used in this embodiment includes an oxide semiconductor film that is highly purified and in which formation of oxygen vacancies are suppressed. The transistor can have low off-state current. Accordingly, an electrical signal such as an image signal can be held for a long time, and a long writing interval can be set in an on state. Accordingly, the frequency of refresh operation can be reduced, which suppresses power consumption.

In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high-speed operation. For example, in a liquid crystal display device that includes such a transistor capable of high-speed operation, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, no additional semiconductor device formed using a silicon wafer or the like is needed as a driver circuit; therefore, the number of components of the semiconductor device can be reduced. In addition, by using the transistor capable of high-speed operation in the pixel portion, a high-quality image can be provided.

The capacitor 790 includes a lower electrode and an upper electrode. The lower electrode is formed through a step of processing the conductive film to be the conductive film functioning as a gate electrode of the transistor 750. The upper electrode is formed through a step of processing the conductive film to be the conductive film functioning as a source electrode or a drain electrode of the transistor 750. Between the lower electrode and the upper electrode, an insulating film formed through a step of forming the insulating film to be the insulating film functioning as a first gate insulating film of the transistor 750 is provided. That is, the capacitor 790 has a stacked-layer structure in which an insulating film functioning as a dielectric film is positioned between the pair of electrodes.

In FIG. 23 to FIG. 25, a planarization insulating film 770 is provided over the transistor 750, the transistor 752, and the capacitor 790.

Although FIG. 23 to FIG. 25 each illustrate an example in which the transistor 750 included in the pixel portion 702 and the transistor 752 included in the source driver circuit portion 704 have the same structure, one embodiment of the present invention is not limited thereto. For example, the pixel portion 702 and the source driver circuit portion 704 may include different transistors. Specifically, a structure in which a top-gate transistor is used in the pixel portion 702 and a bottom-gate transistor is used in the source driver circuit portion 704, or a structure in which a bottom-gate transistor is used in the pixel portion 702 and a top-gate transistor is used in the source driver circuit portion 704 may be employed. Note that the term “source driver circuit portion 704” can be replaced by the term “gate driver circuit portion”.

The signal line 710 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752. In the case where the signal line 710 is formed using a material containing a copper, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, an anisotropic conductive film 780, and the FPC 716. Note that the connection electrode 760 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752. The connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780.

For example, glass substrates can be used as the first substrate 701 and the second substrate 705. As the first substrate 701 and the second substrate 705, flexible substrates may also be used. An example of the flexible substrate is a plastic substrate.

A structure 778 is provided between the first substrate 701 and the second substrate 705. The structure 778 is a columnar spacer obtained by selective etching of an insulating film and is provided to control the distance (cell gap) between the first substrate 701 and the second substrate 705. Alternatively, a spherical spacer may also be used as the structure 778.

A light-blocking film 738 functioning as a black matrix, a coloring film 736 functioning as a color filter, and an insulating film 734 in contact with the light-blocking film 738 and the coloring film 736 are provided on the second substrate 705 side.

<4-2. Structure Example of Display Device Including Liquid Crystal Element>

The display device 700 in FIG. 23 includes a liquid crystal element 775. The liquid crystal element 775 includes a conductive film 772, a conductive film 774, and a liquid crystal layer 776. The conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode. The display device 700 in FIG. 23 can display an image in such a manner that transmission or non-transmission of light is controlled by the alignment state in the liquid crystal layer 776 that is changed depending on the voltage applied between the conductive film 772 and the conductive film 774.

The conductive film 772 is electrically connected to the conductive film functioning as the source electrode or the drain electrode of the transistor 750. The conductive film 772 is formed over the planarization insulating film 770 and functions as a pixel electrode, that is, one electrode of the display element.

A conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film 772. For example, a material containing an element selected from indium (In), zinc (Zn), and tin (Sn) may be used for the conductive film that transmits visible light. For example, a material containing aluminum or silver may be used for the conductive film that reflects visible light.

In the case where a conductive film that reflects visible light is used as the conductive film 772, the display device 700 is a reflective liquid crystal display device. In the case where a conductive film that transmits visible light is used as the conductive film 772, the display device 700 is a transmissive liquid crystal display device.

The method for driving the liquid crystal element can be changed by changing the structure over the conductive film 772, an example of this case is illustrated in FIG. 24. The display device 700 illustrated in FIG. 24 is an example of employing a horizontal electric field mode (e.g., an FFS mode) as a driving mode of the liquid crystal element. In the structure illustrated in FIG. 24, an insulating film 773 is provided over the conductive film 772, and the conductive film 774 is provided over the insulating film 773. In such a structure, the conductive film 774 functions as a common electrode, and an electric field generated between the conductive film 772 and the conductive film 774 through the insulating film 773 can control the alignment state in the liquid crystal layer 776.

Although not illustrated in FIG. 23 and FIG. 24, the conductive film 772 and/or the conductive film 774 may be provided with an alignment film on a side in contact with the liquid crystal layer 776. Although not illustrated in FIG. 23 and FIG. 24, an optical member (optical substrate) or the like, such as a polarizing member, a retardation member, or an anti-reflection member, may be provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. These liquid crystal materials exhibit a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

In the case where a horizontal electric field mode is employed, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. The blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase when the temperature of a cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition containing a liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy, which eliminates the need for an alignment process. An alignment film does not need to be provided, and thus, rubbing treatment is not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented, and defects and damage of a liquid crystal display device in the manufacturing process can be reduced. Moreover, the liquid crystal material that exhibits a blue phase has small viewing angle dependence.

In the case where a liquid crystal element is used as a display element, a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as a vertical alignment (VA) mode transmissive liquid crystal display device may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, and an ASV mode, or the like can be employed.

<4-3. Display Device Including Light-Emitting Element>

The display device 700 illustrated in FIG. 25 includes a light-emitting element 782. The light-emitting element 782 includes a conductive film 772, an EL layer 786, and a conductive film 788. The display device 700 illustrated in FIG. 25 can display an image by utilizing light emission from the EL layer 786 of the light-emitting element 782. Note that the EL layer 786 contains an organic compound or an inorganic compound such as a quantum dot.

Examples of materials that can be used for an organic compound include a fluorescent material and a phosphorescent material. Examples of materials that can be used for a quantum dot include a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, and a core quantum dot material. A material containing elements belonging to Groups 12 and 16, elements belonging to Groups 13 and 15, or elements belonging to Groups 14 and 16, may be used. Alternatively, a quantum dot material containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.

The above-described organic compound and the inorganic compound can be deposited by a method such as an evaporation method (including a vacuum evaporation method), a droplet discharge method (also referred to as an ink-jet method), a coating method, or a gravure printing method. A low molecular material, a middle molecular material (including an oligomer and a dendrimer), or a high molecular material may be included in the EL layer 786.

Here, a method for forming the EL layer 786 by a droplet discharge method is described with reference to FIGS. 28A to 28D. FIGS. 28A to 28D are cross-sectional views illustrating the method for forming the EL layer 786.

First, the conductive film 772 is formed over the planarization insulating film 770, and an insulating film 730 is formed to cover part of the conductive film 772 (see FIG. 28A).

Then, a droplet 784 is discharged to an exposed portion of the conductive film 772, which is an opening of the insulating film 730, from a droplet discharge apparatus 783, so that a layer 785 containing a composition is formed. The droplet 784 is a composition containing a solvent and is attached to the conductive film 772 (see FIG. 28B).

Note that the step of discharging the droplet 784 may be performed under reduced pressure.

Next, the solvent is removed from the layer 785 containing the composition, and the resulting layer is solidified to form the EL layer 786 (see FIG. 28C).

The solvent may be removed by drying or heating.

Next, the conductive film 788 is formed over the EL layer 786; thus, the light-emitting element 782 is formed (see FIG. 28D).

When the EL layer 786 is formed by a droplet discharge method as described above, the composition can be selectively discharged; accordingly, waste of material can be reduced. Furthermore, a lithography process or the like for shaping is not needed, and thus, the process can be simplified and cost reduction can be achieved.

The droplet discharge method described above is a general term for a means including a nozzle equipped with a composition discharge opening or a means to discharge droplets such as a head having one or a plurality of nozzles.

Next, a droplet discharge apparatus used for the droplet discharge method is described with reference to FIG. 29. FIG. 29 is a conceptual diagram illustrating a droplet discharge apparatus 1400.

The droplet discharge apparatus 1400 includes a droplet discharge means 1403. In addition, the droplet discharge means 1403 is equipped with a head 1405 and a head 1412.

The heads 1405 and 1412 are connected to a control means 1407, and this control means 1407 is controlled by a computer 1410; thus, a preprogrammed pattern can be drawn.

The drawing may be conducted at a timing, for example, based on a marker 1411 formed over a substrate 1402. Alternatively, the reference point may be determined on the basis of an outer edge of the substrate 1402. Here, the marker 1411 is detected by an imaging means 1404 and converted into a digital signal by an image processing means 1409. Then, the digital signal is recognized by the computer 1410, and then, a control signal is generated and transmitted to the control means 1407.

An image sensor or the like using a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) can be used as the imaging means 1404. Note that information about a pattern to be formed over the substrate 1402 is stored in a storage medium 1408, and a control signal is transmitted to the control means 1407 based on the information, so that each of the heads 1405 and 1412 of the droplet discharge means 1403 can be individually controlled. The heads 1405 and 1412 are supplied with a material to be discharged from material supply sources 1413 and 1414 through pipes, respectively.

Inside the head 1405, a space as indicated by a dotted line 1406 to be filled with a liquid material and a nozzle which is a discharge outlet are provided. Although it is not shown, an inside structure of the head 1412 is similar to that of the head 1405. When the nozzle sizes of the heads 1405 and 1412 are different from each other, different materials with different widths can be discharged simultaneously. Each head can discharge and draw a plurality of light emitting materials. In the case of drawing over a large area, the same material can be simultaneously discharged to be drawn from a plurality of nozzles in order to improve throughput. When a large substrate is used, the heads 1405 and 1412 can freely scan the substrate in directions indicated by arrows X, Y, and Z in FIG. 29, and a region in which a pattern is drawn can be freely set. Thus, a plurality of the same patterns can be drawn over one substrate.

Further, a step of discharging the composition may be performed under reduced pressure. Also, a substrate may be heated when the composition is discharged. After discharging the composition, either drying or baking or both of them are performed. Both the drying and baking are heat treatments but different in purpose, temperature, and time period. The steps of drying and baking are performed under normal pressure or under reduced pressure by laser irradiation, rapid thermal annealing, heating using a heating furnace, or the like. Note that the timing of the heat treatment and the number of times of the heat treatment are not particularly limited. The temperature for performing each of the steps of drying and baking in a favorable manner depends on the materials of the substrate and the properties of the composition.

In the above-described manner, the EL layer 786 can be formed with the droplet discharge apparatus.

Let's go back to the explanation of the display device 700 illustrated in FIG. 25.

In the display device 700 in FIG. 25, the insulating film 730 is provided over the planarization insulating film 770 and the conductive film 772. The insulating film 730 covers part of the conductive film 772. Note that the light-emitting element 782 has a top-emission structure. Thus, the conductive film 788 has a light-transmitting property and transmits light emitted from the EL layer 786. Although the top-emission structure is described as an example in this embodiment, the structure is not limited thereto. For example, a bottom-emission structure in which light is emitted to the conductive film 772 side or a dual-emission structure in which light is emitted to both the conductive film 772 side and the conductive film 788 side may also be employed.

The coloring film 736 is provided to overlap with the light-emitting element 782, and the light-blocking film 738 is provided in the lead wiring portion 711 and the source driver circuit portion 704 to overlap with the insulating film 730. The coloring film 736 and the light-blocking film 738 are covered with the insulating film 734. A space between the light-emitting element 782 and the insulating film 734 is filled with a sealing film 732. The structure of the display device 700 is not limited to the example in FIG. 25, in which the coloring film 736 is provided. For example, a structure without the coloring film 736 may also be employed in the case where the EL layer 786 is formed by separate coloring.

<4-4. Structure Example of Display Device Provided with Input/Output Device>

An input/output device may be provided in the display device 700 illustrated in FIG. 24 and FIG. 25. As an example of the input/output device, a touch panel or the like can be given.

FIG. 26 illustrates a structure in which the display device 700 in FIG. 24 includes a touch panel 791, and FIG. 27 illustrates a structure in which the display device 700 in FIG. 25 includes a touch panel 791.

FIG. 26 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 24, and FIG. 27 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 25.

First, the touch panel 791 illustrated in FIG. 26 and FIG. 27 is described below.

The touch panel 791 illustrated in FIG. 26 and FIG. 27 is what is called an in-cell touch panel provided between the second substrate 705 and the coloring film 736. The touch panel 791 is formed on the second substrate 705 side before the coloring film 736 is formed.

Note that the touch panel 791 includes the light-blocking film 738, an insulating film 792, an electrode 793, an electrode 794, an insulating film 795, an electrode 796, and an insulating film 797. Changes in the mutual capacitance in the electrodes 793 and 794 can be detected when an object such as a finger or a stylus approaches, for example.

A portion in which the electrode 793 intersects with the electrode 794 is illustrated in the upper portion of the transistor 750 illustrated in FIG. 26 and FIG. 27. The electrode 796 is electrically connected to the two electrodes 793 between which the electrode 794 is sandwiched through openings provided in the insulating film 795. Note that a structure in which a region where the electrode 796 is provided is provided in the pixel portion 702 is illustrated in FIG. 26 and FIG. 27 as an example; however, one embodiment of the present invention is not limited thereto. For example, the region where the electrode 796 is provided may be provided in the source driver circuit portion 704.

The electrode 793 and the electrode 794 are provided in a region overlapping with the light-blocking film 738. As illustrated in FIG. 26, it is preferable that the electrode 793 do not overlap with the light-emitting element 782. As illustrated in FIG. 27, it is preferable that the electrode 793 do not overlap with the liquid crystal element 775. In other words, the electrode 793 has an opening in a region overlapping with the light-emitting element 782 and the liquid crystal element 775. That is, the electrode 793 has a mesh shape. With such a structure, the electrode 793 does not block light emitted from the light-emitting element 782, or alternatively the electrode 793 does not block light transmitted through the liquid crystal element 775. Thus, since luminance is hardly reduced even when the touch panel 791 is provided, a display device with high visibility and low power consumption can be obtained. Note that the electrode 794 can have a structure similar to that of the electrode 793.

Since the electrode 793 and the electrode 794 do not overlap with the light-emitting element 782, a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794. Furthermore, since the electrode 793 and the electrode 794 do not overlap with the liquid crystal element 775, a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794.

Thus, as compared with the case of using an oxide material whose transmittance of visible light is high, resistance of the electrodes 793 and 794 can be reduced, whereby sensitivity of the sensor of the touch panel can be increased.

For example, a conductive nanowire may be used for the electrodes 793, 794, and 796. The nanowire may have a mean diameter of greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, further preferably greater than or equal to 5 nm and less than or equal to 25 nm. As the nanowire, a carbon nanotube or a metal nanowire such as an Ag nanowire, a Cu nanowire, or an Al nanowire may be used. For example, in the case where an Ag nanowire is used for any one of or all of electrodes 793, 794, and 796, the transmittance of visible light can be greater than or equal to 89% and the sheet resistance can be greater than or equal to 40 Ω/sq. and less than or equal to 100 Ω/sq.

Although the structure of the in-cell touch panel is illustrated in FIG. 26 and FIG. 27, one embodiment of the present invention is not limited thereto. For example, a touch panel formed over the display device 700, what is called an on-cell touch panel, or a touch panel attached to the display device 700, what is called an out-cell touch panel may be used.

In this manner, the display device of one embodiment of the present invention can be combined with various types of touch panels.

Note that the structures described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 5

In this embodiment, a display device including a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 30A to 30C.

<5. Circuit Configuration of Display Device>

A display device illustrated in FIG. 30A includes a region including pixels of display elements (hereinafter referred to as a pixel portion 502), a circuit portion that is provided outside the pixel portion 502 and includes a circuit for driving the pixels (hereinafter, the circuit portion is referred to as a driver circuit portion 504), circuits having a function of protecting elements (hereinafter, the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed. Thus, the number of components and the number of terminals can be reduced. When part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X (X is a natural number of 2 or more) rows and Y (Y is a natural number of 2 or more) columns (hereinafter, the circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter referred to as scan lines GL_1 to GL_A). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the gate driver 504 a.

The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (image signal) from which a data signal is generated, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 from the image signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter referred to as data lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the source driver 504 b.

The source driver 504 b includes a plurality of analog switches, for example. The source driver 504 b can output, as data signals, time-divided image signals obtained by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of X or less, and n is a natural number of Y or less), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 506 in FIG. 30A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 refers to a portion having terminals for inputting power, control signals, and image signals from external circuits to the display device.

The protection circuit 506 electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is supplied to the wiring connected to the protection circuit.

As illustrated in FIG. 30A, the protection circuits 506 provided for the pixel portion 502 and the driver circuit portion 504 can improve the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like. Note that the configuration of the protection circuits 506 is not limited thereto; for example, the protection circuit 506 can be connected to the gate driver 504 a or the source driver 504 b. Alternatively, the protection circuit 506 can be connected to the terminal portion 507.

One embodiment of the present invention is not limited to the example in FIG. 30A, in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b. For example, only the gate driver 504 a may be formed, and a separately prepared substrate over which a source driver circuit is formed (e.g., a driver circuit board formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 30A can have the configuration illustrated in FIG. 30B, for example.

The pixel circuit 501 in FIG. 30B includes a liquid crystal element 570, a transistor 550, and a capacitor 560. As the transistor 550, the transistor described in the above embodiment can be used.

The potential of one of a pair of electrodes of the liquid crystal element 570 is set as appropriate in accordance with the specifications of the pixel circuit 501. The alignment state of the liquid crystal element 570 depends on data written thereto. A common potential may be supplied to the one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501. The potential supplied to the one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 may differ between rows.

Examples of a method for driving the display device including the liquid crystal element 570 include a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, and a transverse bend alignment (TBA) mode. Other examples of the method for driving the display device include an electrically controlled birefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Without being limited thereto, various liquid crystal elements and driving methods can be used.

In the pixel circuit 501 in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the data line DL_n, and the other of the source electrode and the drain electrode of the transistor 550 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. A gate electrode of the transistor 550 is electrically connected to the scan line GL_m. The transistor 550 is configured to be turned on or off to control whether a data signal is written.

One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other of the pair of electrodes of the capacitor 560 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. The potential of the potential supply line VL is set as appropriate in accordance with the specifications of the pixel circuit 501. The capacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuits 501 in FIG. 30B, the gate driver 504 a in FIG. 30A sequentially selects the pixel circuits 501 row by row to turn on the transistors 550, and data signals are written.

When the transistor 550 is turned off, the pixel circuit 501 to which the data has been written is brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 30A can have the configuration illustrated in FIG. 30C, for example.

The pixel circuit 501 in FIG. 30C includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. The transistor described in the above embodiment can be used as the transistor 552 and/or the transistor 554.

One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring through which a data signal is supplied (hereinafter referred to as a data line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring through which a gate signal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 is configured to be turned on or off to control whether a data signal is written.

One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other of the pair of electrodes of the capacitor 562 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a. A gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other of the anode and the cathode of the light-emitting element 572 is electrically connected to the other of the source electrode and the drain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element (also referred to as an organic EL element) can be used, for example. Note that the light-emitting element 572 is not limited thereto and may be an inorganic EL element including an inorganic material.

A high power supply potential Von is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential V_(SS) is supplied to the other of the potential supply line VL_a and the potential supply line VL_b.

In the display device including the pixel circuits 501 in FIG. 30C, the gate driver 504 a in FIG. 30A sequentially selects the pixel circuits 501 row by row to turn on the transistors 552, and data signals are written.

When the transistor 552 is turned off, the pixel circuit 501 to which the data has been written is brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

Note that the structures described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 6

In this embodiment, circuit configuration examples to which the transistors described in the above embodiments can be applied are described with reference to FIGS. 31A to 31C, FIGS. 32A to 32C, FIGS. 33A and 33B, and FIGS. 34A and 34B.

Note that in the following description in this embodiment, the transistor including an oxide semiconductor described in the above embodiment is referred to as an OS transistor.

<6. Configuration Example of Inverter Circuit>

FIG. 31A is a circuit diagram of an inverter that can be used for a shift register, a buffer, or the like included in the driver circuit. An inverter 800 outputs a signal whose logic is inverted from the logic of a signal supplied to an input terminal IN to an output terminal OUT. The inverter 800 includes a plurality of OS transistors. A signal Sa can switch electrical characteristics of the OS transistors.

FIG. 31B illustrates an example of the inverter 800. The inverter 800 includes an OS transistor 810 and an OS transistor 820. The inverter 800 can be formed using only n-channel transistors; thus, the inverter 800 can be formed at lower cost than an inverter formed using a complementary metal oxide semiconductor (i.e., a CMOS inverter).

Note that the inverter 800 including the OS transistors can be provided over a CMOS circuit including Si transistors. Since the inverter 800 can be provided so as to overlap with the CMOS circuit, no additional area is required for the inverter 800, and thus, an increase in the circuit area can be suppressed.

Each of the OS transistors 810 and 820 includes a first gate functioning as a front gate, a second gate functioning as a back gate, a first terminal functioning as one of a source and a drain, and a second terminal functioning as the other of the source and the drain.

The first gate of the OS transistor 810 is connected to its second terminal. The second gate of the OS transistor 810 is connected to a wiring that supplies the signal S_(BG). The first terminal of the OS transistor 810 is connected to a wiring that supplies a voltage V_(DD). The second terminal of the OS transistor 810 is connected to the output terminal OUT.

The first gate of the OS transistor 820 is connected to the input terminal IN. The second gate of the OS transistor 820 is connected to the input terminal IN. The first terminal of the OS transistor 820 is connected to the output terminal OUT. The second terminal of the OS transistor 820 is connected to a wiring that supplies a voltage V_(SS).

FIG. 31C is a timing chart illustrating the operation of the inverter 800. The timing chart in FIG. 31C illustrates changes of a signal waveform of the input terminal IN, a signal waveform of the output terminal OUT, a signal waveform of the signal S_(BG), and the threshold voltage of the OS transistor 810.

The signal S_(BG) can be supplied to the second gate of the OS transistor 810 to control the threshold voltage of the OS transistor 810.

The signal S_(BG) includes a voltage V_(BG) _(_) _(A) for shifting the threshold voltage in the negative direction and a voltage V_(BG) _(_) _(B) for shifting the threshold voltage in the positive direction. The threshold voltage of the OS transistor 810 can be shifted in the negative direction to be a threshold voltage V_(TH) _(_) _(A) when the voltage V_(BG) _(_) _(A) is applied to the second gate. The threshold voltage of the OS transistor 810 can be shifted in the positive direction to be a threshold voltage V_(TH) _(_) _(B) when the voltage V_(BG) _(_) _(B) is applied to the second gate.

To visualize the above description, FIG. 32A shows an I_(e)-V_(g) curve, which is one of the electrical characteristics of a transistor.

When a high voltage such as the voltage V_(BG) _(_) _(A) is applied to the second gate, the electrical characteristics of the OS transistor 810 can be shifted to match a curve shown by a dashed line 840 in FIG. 32A. When a low voltage such as the voltage V_(BG) _(_) _(B) is applied to the second gate, the electrical characteristics of the OS transistor 810 can be shifted to match a curve shown by a solid line 841 in FIG. 32A. As shown in FIG. 32A, switching the signal S_(BG) between the voltage V_(BG) _(_) _(A) and the voltage V_(BG) _(_) _(B) enables the threshold voltage of the OS transistor 810 to be shifted in the positive direction or the negative direction.

The shift of the threshold voltage in the positive direction toward the threshold voltage V_(TH) _(_) _(B) can make current less likely to flow in the OS transistor 810. FIG. 32B visualizes this state.

As illustrated in FIG. 32B, a current I_(B) that flows in the OS transistor 810 can be extremely low. Thus, when a signal supplied to the input terminal IN is at a high level and the OS transistor 820 is on (ON), the voltage of the output terminal OUT can drop sharply.

Since a state in which current is less likely to flow in the OS transistor 810 as illustrated in FIG. 32B can be obtained, a signal waveform 831 of the output terminal in the timing chart in FIG. 31C can be made steep. Shoot-through current between the wiring that supplies the voltage V_(DD) and the wiring that supplies the voltage V_(SS) can be low, leading to low-power operation.

The shift of the threshold voltage in the negative direction toward the threshold voltage V_(TH) _(_) _(A) can make current flow easily in the OS transistor 810. FIG. 32C visualizes this state. As illustrated in FIG. 32C, a current I_(A) flowing at this time can be higher than at least the current I_(B). Thus, when a signal supplied to the input terminal IN is at a low level and the OS transistor 820 is off (OFF), the voltage of the output terminal OUT can be increased sharply. Since a state in which current is likely to flow in the OS transistor 810 as illustrated in FIG. 32C can be obtained, a signal waveform 832 of the output terminal in the timing chart in FIG. 31C can be made steep.

Note that the threshold voltage of the OS transistor 810 is preferably controlled by the signal S_(BG) before the state of the OS transistor 820 is switched, i.e., before time T1 or time T2. For example, as in FIG. 31C, it is preferable that the threshold voltage of the OS transistor 810 be switched from the threshold voltage V_(TH) _(_) _(A) to the threshold voltage V_(TH) _(_) _(B) before time TI at which the level of the signal supplied to the input terminal IN is switched to a high level. Moreover, as in FIG. 31C, it is preferable that the threshold voltage of the OS transistor 810 be switched from the threshold voltage V_(TH) _(_) _(B) to the threshold voltage V_(TH) _(_) _(A) before time T2 at which the level of the signal supplied to the input terminal IN is switched to a low level.

Although the timing chart in FIG. 31C illustrates the structure in which the level of the signal SBC is switched in accordance with the signal supplied to the input terminal IN, a different structure may be employed in which voltage for controlling the threshold voltage is held by the second gate of the OS transistor 810 in a floating state, for example. FIG. 33A illustrates an example of such a circuit configuration.

The circuit configuration in FIG. 33A is the same as that in FIG. 31B, except that an OS transistor 850 is added. A first terminal of the OS transistor 850 is connected to the second gate of the OS transistor 810. A second terminal of the OS transistor 850 is connected to a wiring that supplies the voltage V_(BG) _(_) _(B) (or the voltage V_(BG) _(_) _(A)). A first gate of the OS transistor 850 is connected to a wiring that supplies a signal SF. A second gate of the OS transistor 850 is connected to the wiring that supplies the voltage V_(BG) _(_) _(B) (or the voltage V_(BG) _(_) _(A)).

The operation with the circuit configuration in FIG. 33A is described with reference to a timing chart in FIG. 33B.

The voltage for controlling the threshold voltage of the OS transistor 810 is supplied to the second gate of the OS transistor 810 before time T3 at which the level of the signal supplied to the input terminal IN is switched to a high level. The signal SF is set to a high level and the OS transistor 850 is turned on, so that the voltage V_(BG) _(_) _(B) for controlling the threshold voltage is supplied to a node N_(BG).

The OS transistor 850 is turned off after the voltage of the node N_(BG) becomes V_(BG) _(_) _(B). Since the off-state current of the OS transistor 850 is extremely low, the voltage V_(BG) _(_) _(B) held by the node N_(BG) can be retained while the OS transistor 850 remains off. Thus, the number of times the voltage V_(BG) _(_) _(B) is supplied to the second gate of the OS transistor 850 can be reduced and accordingly, the power consumption for rewriting the voltage V_(BG) _(_) _(B) can be reduced.

Although FIG. 31B and FIG. 33A each illustrate the case where the voltage is supplied to the second gate of the OS transistor 810 by control from the outside, a different structure may be employed in which voltage for controlling the threshold voltage is generated on the basis of the signal supplied to the input terminal IN and supplied to the second gate of the OS transistor 810, for example. FIG. 34A illustrates an example of such a circuit configuration.

The circuit configuration in FIG. 34A is the same as that in FIG. 31B, except that a CMOS inverter 860 is provided between the input terminal IN and the second gate of the OS transistor 810. An input terminal of the CMOS inverter 860 is connected to the input terminal IN. An output terminal of the CMOS inverter 860 is connected to the second gate of the OS transistor 810.

The operation with the circuit configuration in FIG. 34A is described with reference to a timing chart in FIG. 34B. The timing chart in FIG. 34B illustrates changes of a signal waveform of the input terminal IN, a signal waveform of the output terminal OUT, an output waveform IN_B of the CMOS inverter 860, and a threshold voltage of the OS transistor 810.

The output waveform IN_B that corresponds to a signal whose logic is inverted from the logic of the signal supplied to the input terminal IN can be used as a signal that controls the threshold voltage of the OS transistor 810. Thus, the threshold voltage of the OS transistor 810 can be controlled as described with reference to FIGS. 32A to 32C. For example, the signal supplied to the input terminal IN is at a high level and the OS transistor 820 is turned on at time T4 in FIG. 34B. At this time, the output waveform IN_B is at a low level. Accordingly, current can be made less likely to flow in the OS transistor 810; thus, the voltage of the output terminal OUT can be sharply decreased.

Moreover, the signal supplied to the input terminal IN is at a low level and the OS transistor 820 is turned off at time T5 in FIG. 34B. At this time, the output waveform IN_B is at a high level. Accordingly, current can easily flow in the OS transistor 810; thus, a rise in the voltage of the output terminal OUT can be made steep.

As described above, in the configuration of the inverter including the OS transistor in this embodiment, the voltage of the back gate is switched in accordance with the logic of the signal supplied to the input terminal IN. In such a configuration, the threshold voltage of the OS transistor can be controlled. The control of the threshold voltage of the OS transistor by the signal supplied to the input terminal IN can cause a steep change in the voltage of the output terminal OUT. Moreover, shoot-through current between the wirings that supply power supply voltages can be reduced. Thus, power consumption can be reduced.

Note that the structures described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 7

In this embodiment, examples of a semiconductor device in which transistors including an oxide semiconductor (OS transistors) described in the above embodiment are used in a plurality of circuits are described with reference to FIGS. 35A to 35E, FIGS. 36A and 36B, FIGS. 37A and 37B, and FIGS. 38A to 38C.

<7. Circuit Configuration Example of Semiconductor Device>

FIG. 35A is a block diagram of a semiconductor device 900. The semiconductor device 900 includes a power supply circuit 901, a circuit 902, a voltage generation circuit 903, a circuit 904, a voltage generation circuit 905, and a circuit 906.

The power supply circuit 901 is a circuit that generates a voltage V_(ORG) used as a reference. The voltage V_(ORG) is not necessarily one voltage and can be a plurality of voltages. The voltage V_(ORG) can be generated on the basis of a voltage V₀ supplied from the outside of the semiconductor device 900. The semiconductor device 900 can generate the voltage V_(ORG) on the basis of one power supply voltage supplied from the outside. Thus, the semiconductor device 900 can operate without the supply of a plurality of power supply voltages from the outside.

The circuits 902, 904, and 906 operate with different power supply voltages. For example, the power supply voltage of the circuit 902 is a voltage applied on the basis of the voltage V_(ORG) and the voltage V_(SS) (V_(ORG)>V_(SS)). For example, the power supply voltage of the circuit 904 is a voltage applied on the basis of a voltage V_(POG) and the voltage V_(SS) (V_(POG)>V_(ORG)). For example, the power supply voltages of the circuit 906 are voltages applied on the basis of the voltage V_(ORG), the voltage V_(SS), and a voltage V_(NEG) (V_(ORG)>V_(SS)>V_(NEG)). When the voltage V_(SS) is equal to a ground potential (GND), the kinds of voltages generated in the power supply circuit 901 can be reduced.

The voltage generation circuit 903 is a circuit that generates the voltage V_(POG). The voltage generation circuit 903 can generate the voltage V_(POG) on the basis of the voltage V_(ORG) supplied from the power supply circuit 901. Thus, the semiconductor device 900 including the circuit 904 can operate on the basis of one power supply voltage supplied from the outside.

The voltage generation circuit 905 is a circuit that generates the voltage V_(NEG). The voltage generation circuit 905 can generate the voltage V_(NEG) on the basis of the voltage V_(ORG) supplied from the power supply circuit 901. Thus, the semiconductor device 900 including the circuit 906 can operate on the basis of one power supply voltage supplied from the outside.

FIG. 35B illustrates an example of the circuit 904 that operates with the voltage V_(POG) and FIG. 35C illustrates an example of a waveform of a signal for operating the circuit 904.

FIG. 35B illustrates a transistor 911. A signal supplied to a gate of the transistor 911 is generated on the basis of, for example, the voltage V_(POG) and the voltage V_(SS). The signal is generated on the basis of the voltage V_(POG) at the time when the transistor 911 is turned on and on the basis of the voltage V_(SS) at the time when the transistor 911 is turned off. As shown in FIG. 35C, the voltage V_(POG) is higher than the voltage V_(ORG). Therefore, a conducting state between a source (S) and a drain (D) of the transistor 911 can be obtained more surely. As a result, the frequency of malfunction of the circuit 904 can be reduced.

FIG. 35D illustrates an example of the circuit 906 that operates with the voltage V_(NEG) and FIG. 35E illustrates an example of a waveform of a signal for operating the circuit 906.

FIG. 35D illustrates a transistor 912 having a back gate. A signal supplied to a gate of the transistor 912 is generated on the basis of, for example, the voltage V_(ORG) and the voltage V_(SS). The signal is generated on the basis of the voltage V_(ORG) at the time when the transistor 912 is turned on and on the basis of the voltage V_(SS) at the time when the transistor 912 is turned off. A signal supplied to the back gate of the transistor 912 is generated on the basis of the voltage V_(NEG). As shown in FIG. 35E, the voltage V_(NEG) is lower than the voltage V_(SS) (GND). Therefore, the threshold voltage of the transistor 912 can be controlled so as to be shifted in the positive direction. Thus, the transistor 912 can be surely turned off and a current flowing between a source (S) and a drain (D) can be reduced. As a result, the frequency of malfunction of the circuit 906 can be reduced and power consumption thereof can be reduced.

The voltage V_(NEG) may be directly supplied to the back gate of the transistor 912. Alternatively, a signal supplied to the gate of the transistor 912 may be generated on the basis of the voltage V_(ORG) and the voltage V_(NEG) and the generated signal may be supplied to the back gate of the transistor 912.

FIGS. 36A and 36B illustrate a modification example of FIGS. 35D and 35E.

In a circuit diagram illustrated in FIG. 36A, a transistor 922 whose conduction state can be controlled by a control circuit 921 is provided between the voltage generation circuit 905 and the circuit 906. The transistor 922 is an n-channel OS transistor. The control signal S_(BG) output from the control circuit 921 is a signal for controlling the conduction state of the transistor 922. Transistors 912A and 912B included in the circuit 906 are the same OS transistors as the transistor 922.

A timing chart in FIG. 36B shows changes in the potential of the control signal S_(BG) and the potential of a node N_(BG). The potential of the node N_(BG) indicates the states of potentials of back gates of the transistors 912A and 912B. When the control signal S_(BG) is at a high level, the transistor 922 is turned on and the voltage of the node N_(BG) becomes the voltage V_(NEG). Then, when the control signal S_(BG) is at a low level, the node N_(BG) is brought into an electrically floating state. Since the transistor 922 is an OS transistor, its off-state current is small. Accordingly, even when the node N_(BG) is in an electrically floating state, the voltage V_(NEG) which has been supplied can be held.

FIG. 37A illustrates an example of a circuit configuration applicable to the above-described voltage generation circuit 903. The voltage generation circuit 903 illustrated in FIG. 37A is a five-stage charge pump including diodes D1 to D5, capacitors C1 to C5, and an inverter INV. A clock signal CLK is supplied to the capacitors C1 to C5 directly or through the inverter INV. When a power supply voltage of the inverter INV is a voltage applied on the basis of the voltage V_(ORG) and the voltage V_(SS), in response to the application of the clock signal CLK, the voltage V_(POG) can be obtained by increasing the voltage V_(ORG) by a voltage five times a potential difference between the voltage V_(ORG) and the voltage V_(SS). Note that a forward voltage of the diodes D1 to D5 is 0 V. A desired voltage V_(POG) can be obtained when the number of stages of the charge pump is changed.

FIG. 37B illustrates an example of a circuit configuration applicable to the above-described voltage generation circuit 905. The voltage generation circuit 905 illustrated in FIG. 37B is a four-stage charge pump including the diodes D1 to D5, the capacitors C1 to C5, and the inverter INV. The clock signal CLK is supplied to the capacitors C1 to C5 directly or through the inverter INV. When a power supply voltage of the inverter INV is a voltage applied on the basis of the voltage V_(ORG) and the voltage V_(SS), in response to the application of the clock signal CLK, the voltage V_(NEG) can be obtained by decreasing the ground voltage, i.e., the voltage V_(SS) by a voltage four times the potential difference between the voltage V_(ORG) and the voltage V_(SS). Note that a forward voltage of the diodes D1 to D5 is 0 V. A desired voltage V_(NEG) can be obtained when the number of stages of the charge pump is changed.

The circuit configuration of the voltage generation circuit 903 is not limited to the configuration of the circuit diagram illustrated in FIG. 37A. Modification examples of the voltage generation circuit 903 are shown in FIGS. 38A to 38C. Note that further modification examples of the voltage generation circuit 903 can be realized by changing voltages supplied to wirings or arrangement of elements in voltage generation circuits 903A to 903C illustrated in FIGS. 38A to 38C.

The voltage generation circuit 903A illustrated in FIG. 38A includes transistors M1 to M10, capacitors C11 to C14, and an inverter INV1. The clock signal CLK is supplied to gates of the transistors M1 to M10 directly or through the inverter INV1. In response to the application of the clock signal CLK, the voltage V_(POG) can be obtained by increasing the voltage V_(ORG) by a voltage four times the potential difference between the voltage V_(ORG) and the voltage V_(SS). A desired voltage V_(POG) can be obtained when the number of stages is changed. In the voltage generation circuit 903A in FIG. 38A, off-state current of each of the transistors M1 to M10 can be low when the transistors M1 to M10 are OS transistors, and leakage of charge held in the capacitors C11 to C14 can be suppressed. Accordingly, raising from the voltage V_(ORG), to the voltage V_(POG) can be efficiently performed.

The voltage generation circuit 903B illustrated in FIG. 38B includes transistors M11 to M14, capacitors C15 and C16, and an inverter INV2. The clock signal CLK is supplied to gates of the transistors M11 to M14 directly or through the inverter INV2. In response to the application of the clock signal CLK, the voltage V_(POG) can be obtained by increasing the voltage V_(ORG) by a voltage twice the potential difference between the voltage V_(ORG) and the voltage V_(SS). In the voltage generation circuit 903B in FIG. 38B, off-state current of each of the transistors M1 to M14 can be low when the transistors M11 to M14 are OS transistors, and leakage of charge held in the capacitors C15 and C16 can be suppressed. Accordingly, raising from the voltage V_(ORG) to the voltage V_(POG) can be efficiently performed.

The voltage generation circuit 903C in FIG. 38C includes an inductor Ind1, a transistor M15, a diode D6, and a capacitor C₁₇. The conduction state of the transistor M15 is controlled by a control signal EN. Owing to the control signal EN, the voltage V_(POG) which is obtained by increasing the voltage V_(ORG) can be obtained. Since the voltage generation circuit 903C in FIG. 38C increases the voltage using the inductor Ind1, the voltage can be increased efficiently.

As described above, in any of the structures of this embodiment, a voltage required for circuits included in a semiconductor device can be internally generated. Thus, in the semiconductor device, the kinds of power supply voltages supplied from the outside can be reduced.

Note that the structures and the like described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 8

In this embodiment, a display module and electronic devices, each of which includes a semiconductor device of one embodiment of the present invention, are described with reference to FIG. 39, FIGS. 40A to 40E, FIGS. 41A to 41G, and FIGS. 42A and 42B.

<8-1. Display Module>

In a display module 7000 illustrated in FIG. 39, a touch panel 7004 connected to an FPC 7003, a display panel 7006 connected to an FPC 7005, a backlight 7007, a frame 7009, a printed board 7010, and a battery 7011 are provided between an upper cover 7001 and a lower cover 7002.

The semiconductor device of one embodiment of the present invention can be used for the display panel 7006, for example.

The shapes and sizes of the upper cover 7001 and the lower cover 7002 can be changed as appropriate in accordance with the sizes of the touch panel 7004 and the display panel 7006.

The touch panel 7004 can be a resistive touch panel or a capacitive touch panel and overlap with the display panel 7006. Alternatively, a counter substrate (sealing substrate) of the display panel 7006 can have a touch panel function. Alternatively, a photosensor may be provided in each pixel of the display panel 7006 to form an optical touch panel.

The backlight 7007 includes a light source 7008. One embodiment of the present invention is not limited to the structure in FIG. 39, in which the light source 7008 is provided over the backlight 7007. For example, a structure in which the light source 7008 is provided at an end portion of the backlight 7007 and a light diffusion plate is further provided may be employed. Note that the backlight 7007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.

The frame 7009 protects the display panel 7006 and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 7010. The frame 7009 may also function as a radiator plate.

The printed board 7010 includes a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the separate battery 7011 may be used. The battery 7011 can be omitted in the case where a commercial power source is used.

The display module 7000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

<8-2. Electronic Device 1>

Next, FIGS. 40A to 40E illustrate examples of electronic devices.

FIG. 40A is an external view of a camera 8000 to which a finder 8100 is attached.

The camera 8000 includes a housing 8001, a display portion 8002, an operation button 8003, a shutter button 8004, and the like. Furthermore, an attachable lens 8006 is attached to the camera 8000.

Although the lens 8006 of the camera 8000 here is detachable from the housing 8001 for replacement, the lens 8006 may be included in the housing 8001.

Images can be taken with the camera 8000 at the press of the shutter button 8004. In addition, images can be taken at the touch of the display portion 8002 that serves as a touch panel.

The housing 8001 of the camera 8000 includes a mount including an electrode, so that the finder 8100, a stroboscope, or the like can be connected to the housing 8001.

The finder 8100 includes a housing 8101, a display portion 8102, a button 8103, and the like.

The housing 8101 includes a mount for engagement with the mount of the camera 8000 so that the finder 8100 can be connected to the camera 8000. The mount includes an electrode, and an image or the like received from the camera 8000 through the electrode can be displayed on the display portion 8102.

The button 8103 serves as a power button. The on/off state of the display portion 8102 can be turned on and off with the button 8103.

A display device of one embodiment of the present invention can be used in the display portion 8002 of the camera 8000 and the display portion 8102 of the finder 8100.

Although the camera 8000 and the finder 8100 are separate and detachable electronic devices in FIG. 40A, the housing 8001 of the camera 8000 may include a finder having a display device.

FIG. 40B is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens 8202, a main body 8203, a display portion 8204, a cable 8205, and the like. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 through the cable 8205. The main body 8203 includes a wireless receiver or the like to receive video data, such as image data, and display it on the display portion 8204. The movement of the eyeball and the eyelid of a user is captured by a camera in the main body 8203 and then coordinates of the points the user looks at are calculated using the captured data to utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as to be in contact with the user. The main body 8203 may be configured to sense current flowing through the electrodes with the movement of the user's eyeball to recognize the direction of his or her eyes. The main body 8203 may be configured to sense current flowing through the electrodes to monitor the user's pulse. The mounting portion 8201 may include sensors, such as a temperature sensor, a pressure sensor, or an acceleration sensor so that the user's biological information can be displayed on the display portion 8204. The main body 8203 may be configured to sense the movement of the user's head or the like to move an image displayed on the display portion 8204 in synchronization with the movement of the user's head or the like.

The display device of one embodiment of the present invention can be used in the display portion 8204.

FIGS. 40C to 40E are external views of a head-mounted display 8300. The head-mounted display 8300 includes a housing 8301, a display portion 8302, a band-shaped object 8304 for fixing the display, and a pair of lenses 8305.

A user can see display on the display portion 8302 through the lenses 8305. It is favorable that the display portion 8302 be curved. When the display portion 8302 is curved, a user can feel high realistic sensation of images. Although the structure described in this embodiment as an example has one display portion 8302, the number of display portions 8302 provided is not limited to one. For example, two display portions 8302 may be provided, in which case one display portion is provided for one corresponding user's eye, so that three-dimensional display using parallax or the like is possible.

The display device of one embodiment of the present invention can be used in the display portion 8302. The display device including the semiconductor device of one embodiment of the present invention has an extremely high resolution; thus, even when an image is magnified using the lenses 8305 as illustrated in FIG. 40E, the user does not perceive pixels, and thus a more realistic image can be displayed.

<8-3. Electronic Device 2>

Next, FIGS. 41A to 41G illustrate examples of electronic devices that are different from those illustrated in FIGS. 40A to 40E.

Electronic devices illustrated in FIGS. 41A to 41G include a housing 9000, a display portion 9001, a speaker 9003, an operation key 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices in FIGS. 41A to 41G have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a recording medium and displaying it on the display portion. Note that functions of the electronic devices in FIGS. 41A to 41G are not limited thereto, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 41A to 41G, the electronic devices may each have a plurality of display portions. Furthermore, the electronic devices may each be provided with a camera and the like to have a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.

The electronic devices in FIGS. 41A to 41G are described in detail below.

FIG. 41A is a perspective view illustrating a television device 9100. The television device 9100 can include the display portion 9001 having a large screen size of, for example, 50 inches or more, or 100 inches or more.

FIG. 41B is a perspective view of a portable information terminal 9101. The portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal 9101 can be used as a smartphone. Note that the portable information terminal 9101 may include a speaker, a connection terminal, a sensor, or the like. The portable information terminal 9101 can display text and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons or simply as icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an e-mail, a social networking service (SNS) message, or a telephone call, the title and sender of an e-mail or an SNS message, date, time, remaining battery, and reception strength of an antenna. Alternatively, the operation buttons 9050 or the like may be displayed in place of the information 9051.

FIG. 41C is a perspective view of a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different surfaces. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) on the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. The user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.

FIG. 41D is a perspective view of a watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. The display surface of the display portion 9001 is curved, and display can be performed on the curved display surface. The portable information terminal 9200 can employ near field communication conformable to a communication standard. For example, hands-free calling can be achieved by mutual communication between the portable information terminal 9200 and a headset capable of wireless communication. Moreover, the portable information terminal 9200 includes the connection terminal 9006 and can perform direct data communication with another information terminal via a connector. Charging through the connection terminal 9006 is also possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

FIGS. 41E, 41F, and 41G are perspective views of a foldable portable information terminal 9201 that is opened, that is shifted from the opened state to the folded state or from the folded state to the opened state, and that is folded, respectively. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined by hinges 9055. By being folded at the hinges 9055 between the two adjacent housings 9000, the portable information terminal 9201 can be reversibly changed in shape from the opened state to the folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature greater than or equal to 1 mm and less than or equal to 150 mm.

Next, an example of an electronic device that is different from the electronic devices illustrated in FIGS. 40A to 40E and FIGS. 41A to 41G is illustrated in FIGS. 42A and 42B. FIGS. 42A and 42B are perspective views of a display device including a plurality of display panels. The plurality of display panels are wound in the perspective view in FIG. 42A and are unwound in the perspective view in FIG. 42B.

A display device 9500 illustrated in FIGS. 42A and 42B includes a plurality of display panels 9501, a hinge 9511, and a bearing 9512. The plurality of display panels 9501 each include a display region 9502 and a light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacent display panels 9501 are provided so as to partly overlap with each other. For example, the light-transmitting regions 9503 of the two adjacent display panels 9501 can overlap with each other. A display device having a large screen can be obtained with the plurality of display panels 9501. The display device is highly versatile because the display panels 9501 can be wound depending on its use.

Although the display regions 9502 of the adjacent display panels 9501 are separated from each other in FIGS. 42A and 42B, without limitation to this structure, the display regions 9502 of the adjacent display panels 9501 may overlap with each other without any space so that a continuous display region 9502 is obtained, for example.

Electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic device that does not have a display portion.

Note that the structures described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2015-244195 filed with Japan Patent Office on Dec. 15, 2015, Japanese Patent Application serial no. 2015-244201 filed with Japan Patent Office on Dec. 15, 2015, Japanese Patent Application serial no. 2016-124845 filed with Japan Patent Office on Jun. 23, 2016, and Japanese Patent Application serial no. 2016-125206 filed with Japan Patent Office on Jun. 24, 2016, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a first gate electrode; a first insulating film over the first gate electrode; an oxide semiconductor film over the first insulating film; and a pair of electrodes over the oxide semiconductor film, wherein the oxide semiconductor film comprises: a first oxide semiconductor film; a second oxide semiconductor film over the first oxide semiconductor film; and a third oxide semiconductor film over the second oxide semiconductor film, wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film comprise the same elements, and wherein the second oxide semiconductor film comprises a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.
 2. The semiconductor device according to claim 1, comprising: a second insulating film over the oxide semiconductor film and the pair of electrodes; and a second gate electrode over the second insulating film.
 3. The semiconductor device according to claim 1, comprising: a second insulating film over the oxide semiconductor film and the pair of electrodes; and a second gate electrode over the second insulating film, wherein the first gate electrode and the second gate electrode are connected through an opening provided in the first insulating film and the second insulating film, and wherein the first gate electrode and the second gate electrode each comprise a region positioned outside an edge portion of the oxide semiconductor film.
 4. The semiconductor device according to claim 1, wherein the second oxide semiconductor film comprises nitrogen.
 5. The semiconductor device according to claim 1, wherein the second oxide semiconductor film comprises a region having a higher nitrogen concentration than the first oxide semiconductor film and the third oxide semiconductor film.
 6. The semiconductor device according to claim 1, wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately comprise indium, M, and zinc, and wherein M is aluminum, gallium, yttrium, or tin.
 7. The semiconductor device according to claim 1, wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately comprise a crystal part, and wherein the crystal part has a c-axis alignment.
 8. A display device comprising: the semiconductor device according to claim 1; and a display element electrically connected to the semiconductor device.
 9. A display module comprising: the display device according to claim 8; and a touch sensor electrically connected to the display device.
 10. An electronic device comprising: the display module according to claim 9; and an operation key or a battery electrically connected to the display module.
 11. A semiconductor device comprising: a first insulating film; an oxide semiconductor film over the first insulating film; a second insulating film and a third insulating film over the oxide semiconductor film; and a first gate electrode over the second insulating film, wherein the oxide semiconductor film comprises: a first region in contact with the first insulating film; a second region in contact with the second insulating film; and a third region in contact with the third insulating film, wherein the first region comprises a region having a higher carrier density than the second region, and wherein the third region comprises a region having a higher carrier density than the second region.
 12. A semiconductor device according to claim 11, comprising a second gate electrode, wherein the first insulating film is positioned over the second gate electrode.
 13. A semiconductor device according to claim 11, comprising a second gate electrode, wherein the first insulating film is positioned over the second gate electrode, wherein the first gate electrode and the second gate electrode are connected through an opening provided in the first insulating film and the second insulating film, and wherein the first gate electrode and the second gate electrode each comprise a region positioned outside an edge portion of the oxide semiconductor film.
 14. The semiconductor device according to claim 11, wherein the first region comprises nitrogen.
 15. The semiconductor device according to claim 11, wherein the first region comprises a region having a higher nitrogen concentration than the second region.
 16. The semiconductor device according to claim 11, wherein the oxide semiconductor film comprises indium, M, and zinc, and wherein M is aluminum, gallium, yttrium, or tin.
 17. The semiconductor device according to claim 11, wherein the oxide semiconductor film comprises a crystal part, and wherein the crystal part has a c-axis alignment.
 18. A display device comprising: the semiconductor device according to claim 11; and a display element electrically connected to the semiconductor device.
 19. A display module comprising: the display device according to claim 18; and a touch sensor electrically connected to the display device.
 20. An electronic device comprising: the display module according to claim 19; and an operation key or a battery electrically connected to the display module. 